Number of embryos for transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection
Abstract
Background
Multiple embryo transfer during in vitro fertilisation (IVF) increases multiple pregnancy rates causing maternal and perinatal morbidity. Single embryo transfer is now being seriously considered as a means of minimising the risk of multiple pregnancy. However, this needs to be balanced against the risk of jeopardising the overall live birth rate.
Objectives
To evaluate the effectiveness and safety of different policies for the number of embryos transferred in couples who undergo assisted reproductive technology (ART).
Search methods
We searched the Cochrane Menstrual Disorders and Subfertility Group Trials Register, the Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE and EMBASE, from inception to July 2013. We handsearched reference lists of articles, trial registers and relevant conference proceedings and contacted researchers in the field.
Selection criteria
We included randomised controlled trials (RCTs) comparing different policies for the number of embryos transferred following IVF or intra‐cytoplasmic sperm injection (ICSI) in subfertile women. Studies of fresh or frozen and thawed transfer of one, two, three or four embryos at cleavage or blastocyst stage were eligible.
Data collection and analysis
Two review authors independently assessed trial eligibility and risk of bias and extracted the data. The overall quality of the evidence was graded in a summary of findings table.
Main results
Fourteen RCTs were included in the review (2165 women). Thirteen compared cleavage‐stage transfers (2017 women) and two compared blastocyst transfers (148 women): one study compared both. No studies compared repeated single versus repeated multiple embryo transfer (SET).
Repeated SET versus DET
Repeated SET was compared with DET in three studies of cleavage‐stage transfer. In these studies the SET group received either two cycles of fresh SET (one study) or one cycle of fresh SET followed by one frozen SET in a natural or hormone‐stimulated cycle (two studies). When these three studies were pooled, the cumulative live birth rate after repeated SET was not significantly different from the rate after one cycle of DET (OR 0.82, 95% CI 0.62 to 1.09, three studies, n=811, I2=0%, low quality evidence). This suggests that for a woman with a 42% chance of live birth following a single cycle of DET, the chance following repeated SET would be between 31% and 44%. The multiple pregnancy rate was significantly lower in the SET group (OR 0.03, 95% CI 0.01 to 0.13, three RCTs, n = 811, I2 = 23%, low quality evidence), suggesting that for a woman with a 13% risk of multiple pregnancy following a single cycle of DET, the risk following repeated SET would be between 0% and 2%.
Single‐cycle SET versus single‐cycle DET
A single cycle of SET was compared with a single cycle of DET in 10 studies, nine comparing cleavage‐stage transfers and two comparing blastocyst‐stage transfers. When studies were pooled the live birth rate was significantly lower in the SET group (OR 0.48, 95% CI 0.39 to 0.60, nine studies, n = 1564, I2 = 0%, high quality evidence). This suggests that for a woman with a 45% chance of live birth following a single cycle of DET, the chance following a single cycle of SET would be between 24% and 33%. The multiple pregnancy rate was also significantly lower in the SET group (OR 0.12, 95% CI 0.07 to 0.20, 10 studies, n = 1612, I2 = 45%, high quality evidence), suggesting that for a woman with a 14% risk of multiple pregnancy following a single cycle of DET, the risk following a single cycle of SET would be between 1% and 3%. The heterogeneity for this analysis was attributable to a study with a high rate of cross‐over between treatment arms.
Other comparisons
Other comparisons were evaluated in four studies which compared DET versus transfer of three or four embryos. Live birth rates did not differ significantly between the groups for any comparison, but there was a significantly lower multiple pregnancy rate in the DET group than in the three embryo transfer (TET) group (OR 0.36, 95% CI 0.13 to 0.99, two studies, n = 343, I2 = 0%).
Authors' conclusions
In a single fresh IVF cycle, single embryo transfer is associated with a lower live birth rate than double embryo transfer. However, there is no evidence of a significant difference in the cumulative live birth rate when a single cycle of double embryo transfer is compared with repeated SET (either two cycles of fresh SET or one cycle of fresh SET followed by one frozen SET in a natural or hormone‐stimulated cycle). Single embryo transfer is associated with much lower rates of multiple pregnancy than other embryo transfer policies. A policy of repeated SET may minimise the risk of multiple pregnancy in couples undergoing ART without substantially reducing the likelihood of achieving a live birth. Most of the evidence currently available concerns younger women with a good prognosis.
PICOs
Plain language summary
Number of embryos for transfer in women undergoing assisted reproductive technology (ART)
Review question:
How many embryos should be transferred in couples undergoing ART?
Background:
Multiple pregnancy creates serious health risks for the mother (such as premature labour, diabetes and high blood pressure) and for the babies, who are at much higher risk than single babies of problems including premature birth, low birth weight, cerebral palsy and perinatal death. Single embryo transfer is now being seriously considered in order to reduce multiple pregnancies but this needs to be balanced against the risk of lowering the overall live birth rate. Researchers in The Cochrane Collaboration reviewed the evidence about the number of embryos transferred in women undergoing ART. The search is current to July 2013.
Study characteristics:
We found 14 randomised controlled trials with a total of 2165 participants. Most were not commercially funded.
Key findings:
Double versus repeated single embryo transfer
Based on low quality evidence, there was no indication that overall live birth rates differed substantially when repeated single embryo transfer (either two cycles of single embryo transfer or one cycle of single embryo transfer followed by transfer of a single frozen embryo in a natural or hormone‐stimulated cycle) was compared with double embryo transfer. The evidence suggested that for a woman with a 42% chance of live birth following a single cycle of double embryo transfer, the chance following repeated single embryo transfer would be between 31% and 44%. The risk of multiple birth was very much lower in the single embryo transfer group: for a woman with a 13% risk of multiple pregnancy following a single cycle of double embryo transfer, the estimated risk following a repeated single transfer was between 0% and 2%.
Double versus single embryo transfer
We found high quality evidence that the chances of live birth were lower after one cycle of fresh single embryo transfer than after one cycle of fresh double embryo transfer. For a woman with a 45% chance of live birth following a single cycle of double embryo transfer, the chance following a single cycle of single embryo transfer was between 24% and 33%. However, the risk of twins was about seven times higher after double embryo transfer.
Conclusion:
Repeated single embryo transfer appears the best option for most women undergoing ART. Most of the evidence currently available concerns younger women with a good prognosis.
Authors' conclusions
Summary of findings
| Repeated single compared to mixed policies for transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Population: women having embryo transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Outcomes | Illustrative comparative risks* (95% CI) | Relative effect | No of Participants | Quality of the evidence | Comments | |
| Assumed risk | Corresponding risk | |||||
| Double ET | Repeated single ET | |||||
| Cumulative live birth Repeated single embryo transfer versus double embryo transfer | 420 per 1000 | 373 per 1000 | OR 0.82 (0.62 to 1.09 ) | 811 | ⊕⊕⊝⊝ | |
| Cumulative live birth ‐ Single embryo transfer plus one cycle of frozen embryo transfer versus one cycle of double embryo transfer | 422 per 1000 | 377 per 1000 | OR 0.83 (0.61 to 1.12 ) | 703 | ⊕⊕⊝⊝ | |
| Cumulative live birth ‐ Two cycles of single embryo transfer SET (x2) versus one cycle of double embryo transfer | 407 per 1000 | 352 per 1000 | OR 0.79 (0.36 to 1.72 ) | 108 | ⊕⊝⊝⊝ | |
| Multiple pregnancy Repeated single embryo transfer versus double embryo transfer | 133 per 1000 | 5 per 1000 | OR 0.03 (0.01 to 0.13 ) | 811 | ⊕⊕⊝⊝ | |
| Multiple pregnancy Single embryo transfer plus one cycle of frozen embryo transfer versus one cycle of double embryo transfer | 136 per 1000 | 5 per 1000 | OR 0.03 (0.01 to 0.14 ) | 703 | ⊕⊕⊝⊝ | |
| Multiple pregnancy Two cycles of single embryo transfer SET (x2) versus one cycle of double embryo transfer | 111 per 1000 | 9 per 1000 | OR 0.07 (0.00 to 1.25 ) | 108 | ⊕⊝⊝⊝ | |
| *The basis for the assumed risk is the median control group risk across studies. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). | ||||||
| GRADE Working Group grades of evidence | ||||||
| 1Methods of allocation concealment not described in enough detail 2Wide confidence intervals 3One small study | ||||||
| Single compared to multiple embryo transfer (in a single cycle) following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Population: women having embryo transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Outcomes | Illustrative comparative risks* (95% CI) | Relative effect | No of Participants | Quality of the evidence | Comments | |
| Assumed risk | Corresponding risk | |||||
| Multiple | Single | |||||
| Multiple pregnancy | 144 per 1000 | 20 per 1000 | OR 0.12 (0.07 to 0.20 ) | 1612 | ⊕⊕⊕⊕ | |
| Live birth | 450 per 1000 | 282 per 1000 | OR 0.48 (0.39 to 0.60 ) | 1564 | ⊕⊕⊕⊕ | |
| *The basis for the assumed risk is the median control group risk across studies). The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). | ||||||
| GRADE Working Group grades of evidence | ||||||
| 136% of women noncompliant with treatment allocation in one study. However, no statistical heterogeneity detected (I2=0%) 2Moderate heterogeneity attributable to 36% of women oncompliant with treatment allocation in one study (I2=45%) | ||||||
Background
Description of the condition
Historically, in an effort to achieve 'acceptable' pregnancy rates, most women undergoing in vitro fertilisation (IVF) have received transfer of multiple embryos. However, this practice is being reassessed due to the high rates of multiple pregnancy that result from multiple embryo transfer and which commonly lead to poor clinical outcomes for the mother or her children, or both (ASRM 2012).
In the 1990s it was calculated that women undergoing IVF had an approximately 20‐fold increased risk of twins and 400‐fold increased risk of higher order pregnancies (Martin 1998). In 2006, twins accounted for nearly 20% of all live births resulting from IVF in Europe (De Mouzon 2010). Widespread concern about the medical, social and economic consequences of multiple pregnancy has prompted the development of strategies aimed at promoting birth of a single healthy baby following IVF.
Compared with singleton births, twins have a four‐fold increased risk of mortality, and for triplets the risk is increased six‐fold (ESHRE 2000). A recent study (ESHRE 2012) of 50,258 births following IVF and intra‐cytoplasmic sperm injection (ICSI) pregnancies reported that twins accounted for half the total neonatal deaths and one‐third of the perinatal deaths. Twins had a significantly higher perinatal mortality rate than singletons (27.8 per 1000 births and 12.4 per 1000 births, respectively). The relatively high congenital malformation rates observed in babies born after IVF and intracytoplasmic sperm injection (ICSI) are attributed to the high proportion of multiple births in this population compared to the general population (Sebire 2000; Wennerholm 2000). In babies with very low birth weight, twin gestation is an independent risk factor for neurodevelopmental impairment including cerebral palsy, severe bilateral hearing loss and bilateral blindness (Wadhawan 2009).
Twin pregnancy also increases the risk of obstetric complications, with a high incidence of miscarriage, pregnancy‐induced hypertension, gestational diabetes, premature labour and abnormal delivery (FIVNAT 1995; ESHRE 2000). After the initial sense of achievement of parenthood, the care of children from a multiple gestation is often associated with practical difficulties and high stress levels (Garel 1992; Doyle 1996; Garel 1997). More hours per week are required to care for six‐month old triplets and to carry out the necessary household tasks. Even in families with material resources and plenty of help, emotional stress is not uncommon and may necessitate psychiatric help (Garel 1997).
The economic impact of multiple pregnancies on health services is another consideration. In an Australian study, the average cost of an ART twin delivery was almost three times as high as for an ART singleton, while for higher order multiple births the cost was up to 11 times greater (Chambers 2007). It has been suggested that redeployment of money saved by reduction of multiple pregnancies could allow for increased provision of IVF treatment in the UK at no extra cost (Ledger 2006).
Description of the intervention
IVF or ICSI is followed by the transfer of one, two, three or four fresh or frozen and thawed embryos. Unused embryos can be frozen and transferred in a subsequent natural or hormone stimulated transfer cycle. Reduction of the number of embryos transferred is a strategy used to reduce rates of multiple pregnancy associated with ART.
There is a worldwide trend for an increase in the rates of elective single embryo transfer, defined as the transfer of a single embryo at cleavage or blastocyst stage, which is chosen from a larger number of available embryos. In Europe, in 2005, about 20% of all embryo transfers were of single embryos but much higher rates are reported in some countries (69% in Sweden in 2005, and 57% in Australia and New Zealand in 2006) (ASRM 2012).
Embryos are often transferred after culture for two or three days, when they comprise two to eight cells (cleavage stage). The rationale for cleavage‐stage transfer is that the uterus is the best environment for the survival of the embryo (Laverge 2001). Over the past decade there has been a steady shift in practice to the transfer of embryos on day five or six, when they have developed into blastocysts with 64 cells. Blastocyst transfer has been shown to be successful (Papanikolaou 2006; Khalaf 2008) but requires laboratory expertise and experience in extended embryo culture. An advantage of blastocyst transfer is that embryos surviving five days are more likely to be viable than embryos at two or three days, and so the likelihood of implantation is higher. Disadvantages of blastocyst transfer include a higher risk of cycles being cancelled (Marek 1999) and fewer embryos being available for cryopreservation due to failed embryo development.
A Cochrane review comparing cleavage‐stage versus blastocyst transfer (Glujovsky 2012) had mixed findings. There was evidence that blastocyst transfer was associated with a small but significant benefit in the live birth rate per couple but that cleavage‐stage transfers were associated with higher cumulative clinical pregnancy rates. This finding was attributed to higher rates of frozen embryos and lower failure‐to‐transfer rates obtained from cleavage‐stage protocols. Multiple birth rates did not differ between the two groups.
How the intervention might work
A strategy of reducing the risk of multiple pregnancy by limiting the number of embryos transferred needs to be balanced against the risk of jeopardising the overall pregnancy rate. An obvious solution is to consider an individualised embryo transfer policy based on identification of key clinical and laboratory parameters associated with a higher implantation rate. The above‐mentioned ESHRE study (ESHRE 2012) of 50,258 births following IVF and ICSI pregnancies reported that double embryo transfer was associated with a 53% higher risk of perinatal mortality than single embryo transfer (19 per 1000 births compared with 13 per 1000 births). This difference was especially apparent when fresh (unfrozen) embryos were used. Births following the transfer of two fresh embryos had a 74% higher risk of perinatal mortality than those following fresh single embryo transfer.
Use of elective single embryo transfer at the cleavage stage (day two or three) has been limited in clinical practice for fear that the overall success rates of IVF would decline. This assumption has been supported by the published results of single embryo transfer where only one embryo was available. Because no opportunity for selection of more suitable embryos exists, the implantation potential of the only available embryo is usually poor, with clinical pregnancy rates of around 10% (FIVNAT 1995; Giorgetti 1995; Preutthipan 1996; Yaron 1997; Lieberman 1998; Westergaard 2000). In a situation where the transferred embryos are the only available embryos, pregnancy rates are unfavourable even for multiple embryo transfer (Ludwig 2000).
A study from Finland reported a 20.2% pregnancy rate in 94 women who had only one embryo available for transfer compared with a rate of 29.7% in women who had multiple embryos available and from which a single high quality embryo was selected for transfer. The cumulative pregnancy rate after frozen and thawed embryo transfers in the elective single embryo transfer group was 47.3% per oocyte retrieval. By comparison, the pregnancy rate for double embryo transfers was 29.4% per transfer, of which 23.9% were twin pregnancies (Vilska 1999).
Another strategy for reducing multiple pregnancy is multifetal pregnancy reduction. However, this procedure is invasive, can have long term adverse psychological consequences for the potential parents (Berkowits 1996; McKinney 1996) and may be unacceptable to some couples given the attendant ethical and legal issues. Clinicians in Europe have generally accepted the desirability of reducing multiple births by limiting the number of embryos transferred, especially if this can be achieved without unduly reducing live birth rates (Roberts 2011).
Why it is important to do this review
It is important to find ways to limit the risk of multiple pregnancy without reducing the chance of achieving live birth in couples undergoing ART cycles. This systematic review evaluates the effectiveness and safety of different policies for the number of embryos transferred in couples who undergo assisted reproductive technology (ART).
Objectives
To evaluate the effectiveness and safety of different policies for the number of embryos transferred in couples undergoing assisted reproductive technology (ART) cycles.
Methods
Criteria for considering studies for this review
Types of studies
Published and unpublished randomised controlled trials (RCTs) were eligible for inclusion. We excluded non‐randomised studies (for example studies with evidence of inadequate sequence generation such as alternate days, chart numbers) as they are associated with a high risk of bias. Cross‐over trials were eligible but it was planned that only data from the first phase would be included in the meta‐analysis as the cross‐over design is not valid in this context.
Types of participants
Trials of subfertile women who underwent embryo transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection treatment with their own gametes or as oocyte or embryo donation recipients were eligible for inclusion.
Types of interventions
We compared the following interventions.
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Repeated single embryo transfer versus repeated multiple transfer.
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Repeated single embryo transfer versus mixed policies
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Single versus multiple embryo transfer in a single cycle.
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Other fresh cycle comparisons.
It was required that elective transfer of embryos followed an initial fresh IVF or ICSI treatment using standard protocols for controlled ovarian stimulation, oocyte retrieval under ultrasound guidance, insemination, embryo culture, and transcervical replacement of embryos (cleavage stage or blastocyst) using standard culture medium and catheters for the culture and transfer of embryos respectively.
Studies could (in addition) transfer one or more frozen thawed embryos in one or both arms using standard procedures in a natural or hormone‐stimulated cycle.
Studies comparing cleavage‐stage transfer versus blastocyst‐stage transfer were excluded.
Types of outcome measures
Primary outcome
(1) Live birth rate per woman or couple, or cumulative live birth rate per woman or couple (in trials with multiple transfers or multiple cycles).
Live birth was defined as the delivery of one or more living infants. Cumulative live birth rate reflects the number of live births following fresh and frozen embryo transfers after a single IVF treatment leading to the harvesting of eggs, or (where stated) after multiple IVF cycles. It is calculated by dividing the total number of live births in each group by the total number of women randomised in each group. One IVF cycle is defined as a single treatment leading to the harvesting of eggs.
(2) Multiple pregnancy rate per woman or couple. The demonstration of more than one sac with a fetal pole on ultrasound scan defines a multiple pregnancy.
Secondary outcomes
(1) Pregnancy rate per woman or couple.
Pregnancy was defined as the presence of a gestational sac on ultrasound scan or confirmation of products of conception by pathological examination in the event of spontaneous abortion or ectopic pregnancy.
(2) Miscarriage rate per woman.
Search methods for identification of studies
We searched for all relevant published and unpublished RCTs without language restriction and in consultation with the Menstrual Disorders and Subfertility Group (MDSG) Trials Search Co‐ordinator. For the search strategies, please see Appendix 1, Appendix 2, Appendix 3, Appendix 4, Appendix 5, Appendix 6.
Electronic searches
We searched the following electronic databases: the Menstrual Disorders and Subfertility Group (MDSG) Specialised Register of controlled trials, Cochrane Central Register of Controlled Trials (CENTRAL), MEDLINE, EMBASE, PsycINFO and CINAHL. The last search date was July 17th 2013.
Other electronic sources of trials included the following.
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Trials registers for ongoing and registered trials:
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www.clinicaltrials.gov/;
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www.who.int/trialsearch/Default.aspx.
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OpenGrey for unpublished literature from Europe at www.opengrey.eu/.
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Citation index: Web of Science.
Searching other resources
We handsearched other resources as follows:
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conference proceedings ‐ International Federation of Fertility Societies (IFFS), American Society for Reproductive Medicine (ASRM), British Fertility Society (BFS), European Society for Human Reproduction and Embryology (ESHRE) between 1997 and 2013;
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the bibliographies of the identified studies.
We personally communicated with experts and investigators in the field.
Data collection and analysis
Selection of studies
The selection of trials for inclusion in the review from those identified employing the search strategy was performed independently by at least two review authors. Disagreements about study eligibility were resolved by discussion.
Data extraction and management
Quality assessment and data extraction were independently performed by two review authors. Any discrepancies were resolved by discussion with senior review authors (GS, SB). Additional information on trial methodology or trial data was sought from the principal authors of trials which appeared to meet the eligibility criteria but were unclear in aspects of methodology, or where the data were in a form unsuitable for meta‐analysis.
Assessment of risk of bias in included studies
The included studies were assessed for risk of bias using the Cochrane risk of bias tool to evaluate the following: random sequence generation; allocation concealment; blinding of participants, providers and outcome assessors; completeness of outcome data; selective outcome reporting; and other potential sources of bias (see Figure 1). At least two authors (ZP, SB, JM) assessed these six domains. Any disagreements were resolved by consensus or by discussion with another author. The assessments are presented in the 'Risk of bias' table (see Characteristics of included studies,Figure 1 and Figure 2).
Methodological quality graph: review authors' judgements about each methodological quality item presented as percentages across all included studies.
Methodological quality summary: review authors' judgements about each methodological quality item for each included study.
Measures of treatment effect
All data were dichotomous. The numbers of events in the control and intervention groups of each study were used to calculate the Mantel‐Haenszel odds ratios (ORs) with 95% confidence intervals (CIs).
Where outcome data were reported as a percentage of the total number of participants, they were included in the analyses by multiplying the percentage by the total number of participants (n) in that group and dividing by 100.
Unit of analysis issues
Multiple live births (for example twins or triplets) were counted as one live birth event. It was planned to include only first‐phase data from cross‐over trials. Per cycle data were not included in tables of comparison but were reported descriptively.
Dealing with missing data
The data were analysed on an intention‐to‐treat basis as far as possible and attempts were made to obtain missing data from the original investigators.
Assessment of heterogeneity
The authors considered whether the clinical and methodological characteristics of the included studies were sufficiently similar for meta‐analysis to provide a meaningful summary. Clinical heterogeneity in subfertility (such as variations in entry criteria, subtle differences in the treatment used and that are important from a clinical aspect) cannot be avoided because most centres use their own protocols which can vary in some aspects. When trials met the inclusion criteria and had performed the same intervention we considered it appropriate to pool their results. Statistical heterogeneity was assessed by inspecting the scatter in the data points and the overlap in their CIs and, more formally by checking the results of the I2 statistic. An I2 measurement greater than 50% was taken to indicate substantial heterogeneity (Higgins 2011). If substantial heterogeneity was detected, possible explanations were explored in sensitivity analyses. Even when included trials in a comparison group were statistically homogeneous, there were potentially considerable differences in clinical features (clinical heterogeneity). These differences were taken into account when analysing and interpreting the pooled results.
Assessment of reporting biases
In view of the difficulty of detecting and correcting for publication bias and other reporting biases, we aimed to minimise their potential impact by ensuring a comprehensive search for eligible studies and by being alert for duplication of data. If there were sufficient studies (preferably more than 10) for the primary outcomes, we planned to use a funnel plot to explore the possibility of small study effects (a tendency for estimates of the intervention effect to be more beneficial in smaller studies).
Data synthesis
The data from primary studies were combined with RevMan software to calculate pooled Mantel‐Haenszel ORs and 95% CIs, using a fixed‐effect model, with the following comparisons.
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Repeated single versus repeated multiple transfer.
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Repeated single embryo transfer versus mixed policies
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Single versus multiple embryo transfer in a single cycle
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Other fresh cycle comparisons
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Other fresh or frozen cycle comparisons
Data were stratified by the stage of embryo transfer (cleavage or blastocyst).
For the 2012 update, we reformatted the comparisons of interest, as above. The choice of repeated single versus repeated multiple embryo transfer as the first comparison of interest reflects the view that a policy of repeated SET may optimise the chance of live birth while minimising the risk of multiple pregnancy (Roberts 2011).
An increase in the odds of a particular outcome, which may be beneficial (for example live birth) or detrimental (for example multiple pregnancy), is displayed graphically in the meta‐analyses to the right of the centre‐line and a decrease in the odds of an outcome to the left of the centre‐line.
Subgroup analysis and investigation of heterogeneity
If data were available, we planned to conduct subgroup analyses to determine the separate evidence within groups with different prognostic characteristics.
If we detected substantial heterogeneity, we planned to explore possible explanations in sensitivity analyses. We planned to take any statistical heterogeneity into account when interpreting the results.
Sensitivity analysis
We conducted sensitivity analyses for the primary outcomes to determine whether the conclusions were robust to arbitrary decisions made regarding study eligibility and statistical methods. We considered whether the review conclusions would have differed if:
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eligibility was restricted to studies at lower risk of bias (i.e. with clearly reported methods of randomisation and allocation concealment and not at high risk of bias in any of the domains assessed);
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a random‐effects model had been adopted;
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the summary effect measure had been relative risk rather than odds ratio (OR).
Overall quality of the body of evidence: 'Summary of findings' table
A 'Summary of findings' table was generated using the GRADEPro software. This table evaluated the overall quality of the body of evidence for the primary review outcomes for selected comparisons. Items assessed were study limitations (that is risk of bias), consistency of effect, imprecision, indirectness and publication bias. Judgements about evidence quality (high, moderate or low) were incorporated into the reporting of results.
Results
Description of studies
Results of the search
The search for the 2013 update identified 640 articles (including duplicates) of which 14 full text articles or online abstracts were retained for detailed appraisal. Five of the 14 were included in the review (ASSETT 2003; Gardner 2004; Thurin 2005; ECOSSE 2006; Fernandez‐Sanchez 2012), six were excluded (Motta 1998 A & B; Livingstone 2001; Bowman 2004; Elgindy 2011; Guerif 2011; Forman 2012), one is awaiting assessment (Obrado 2012) and two are ongoing (Abuzeid 2012; Scott 2013). In addition, two studies excluded from the previous version of the review were included (Komori 2004; Mostajeran 2006). For details, see Figure 3.
Study flow diagram.
Included studies
For this update, seven studies were added to the seven included in the original review, making a total of 14 included studies. Five new studies (ASSETT 2003; Gardner 2004; Thurin 2005; ECOSSE 2006; Fernandez‐Sanchez 2012) were added. Two previously excluded studies (Komori 2004; Mostajeran 2006) were also added. These two studies had been excluded from the previous version of the review for failure to report full details of randomisation and allocation concealment. They were added to this update after discussion between the review authors, who noted that poor reporting was not a review exclusion criterion. Additional information was sought from authors of all the new trials and replies were received from four (ASSETT 2003; Thurin 2005; ECOSSE 2006; Fernandez‐Sanchez 2012). See the 'Characteristics of included studies' table.
Study design and setting
Fourteen studies with a total of 2165 participants were included in the review (Vauthier‐Brouzes 1994; Gerris 1999; Martikainen 2001; ASSETT 2003; Gardner 2004; Komori 2004; Thurin 2004; Lukassen 2005; Thurin 2005; ECOSSE 2006; Heijnen 2006; Mostajeran 2006; van Montfoort 2006; Fernandez‐Sanchez 2012). All were randomised parallel‐group trials. Six were multicentred (Martikainen 2001; ASSETT 2003; Thurin 2004; Thurin 2005; ECOSSE 2006; Heijnen 2006). Sample sizes ranged from 23 to 661 women.
Of the four unpublished studies that have been added to this update, one was a pilot trial published as part of a PhD dissertation (Thurin 2005). Another, the 'Australian study of single embryo transfer' (ASSETT 2003) was stopped early because its implementation immediately and substantially altered consumer decision making. This had the effect of more than tripling rates of elective single embryo transfer during the study period and reducing study participation rates (M Davies, University of Adelaide, personal communication). A UK trial, known as the 'Efficacy and cost effectiveness of selective single embryo transfer' (ECOSSE) study, was also stopped early due to poor recruitment (ECOSSE 2006). The fourth unpublished study (Fernandez‐Sanchez 2012) was in press.
Nine studies reported their funding sources. Six reported non‐commercial funding (Gerris 1999; ASSETT 2003; ECOSSE 2006; Mostajeran 2006; van Montfoort 2006; Fernandez‐Sanchez 2012) and three reported pharmaceutical company funding (Gardner 2004; Thurin 2004; Thurin 2005).
Participants
Study inclusion criteria differed with regard to participant age. Most studies had a maximum age threshold. This varied across studies and included 34 years (Gerris 1999), 35 years (Vauthier‐Brouzes 1994; Lukassen 2005), 36 years (Thurin 2004), 38 years (ECOSSE 2006; Fernandez‐Sanchez 2012), and 40 years (ASSETT 2003). One study included women aged between 38 and 45 years (Heijnen 2006) while another required them to be at least 36 years old (Thurin 2005). Other studies used a variety of age limits (Martikainen 2001; van Montfoort 2006).
Two studies only included women in their first treatment cycle (Gerris 1999; van Montfoort 2006) while three included women with an indication for IVF or ICSI either for the first time or after a previous successful treatment (Vauthier‐Brouzes 1994; Lukassen 2005; Heijnen 2006). Three studies included women in their first or second IVF or ICSI treatment cycle (ASSETT 2003;Thurin 2004; Thurin 2005). In a multicentre study, one centre included women in their first treatment cycle only and another centre included women in their first or second cycle (Martikainen 2001). One study included all women undergoing IVF and embryo transfer (Gardner 2004) who agreed to participate.
The duration of infertility was mentioned in six studies (Gerris 1999; Thurin 2004; Lukassen 2005; Thurin 2005; Heijnen 2006; van Montfoort 2006) and seven mentioned the indication(s) for treatment (Martikainen 2001; Thurin 2004; Lukassen 2005; Thurin 2005; Heijnen 2006; Mostajeran 2006; van Montfoort 2006). See 'Prognostic factors' in Table 1.
| Study author and year | Age Eligibility criteria (mean participant age, where stated) | Duration of infertility | Previous failed cycle | Frozen cycles | Prim/Sec infertility | FSH | Quality of embryo |
| Fernandez‐Sanchez 2012 | Under 38 years (mean age 33) | Mean 2.6 to 3.2 years | First IVF/ICSI cycle. | Frozen cycles included | Not stated | Not stated | good |
| Gerris 1999 | less than 34 years | Average duration of infertility 3.5 years. | First IVF/ICSI cycle. | Not included | unclear | not mentioned | good |
| Heijnen 2007 | 38‐45 years (mean age 41) | Average duration of infertility in DET group was 3.7(+/‐ 2.5) and in TET group was 3.2(+/‐ 2.4) | First cycle and previous successful cycle | Not included | yes | not mentioned | good |
| Komori 2004 | Not stated | Not stated | Not stated | Not stated | Not stated | Not stated | Good |
| Lukassen 2005 | <35 years (mean age 30‐31) | not stated | First IVF/ICSI cycle or after previous successful cycle . | Not included | yes | FSH < 10IU/L. | good |
| Martikainen 2001 | various, no age criteria, ranged between 22‐40 years (mean age 31) | not stated | women who had / not had more than one previous failed treatment. | Frozen cycles included | yes, but not mentioned | not mentioned | good |
| Mostajeran 2006 | Not stated | Not stated | Not stated | Not stated | Not stated | Not stated | Good |
| Thurin 2004 | <36 years (mean age 31) | 0‐12 years | First or second IVF cycle | Frozen cycles included | yes | not mentioned | good, blastocysts included |
| Thurin 2005 Unpublished trial, pilot study, part of a thesis | ≥36 years | 0‐12 years | First or second IVF/ICSI cycle | Frozen cycles included | yes | not mentioned | At |
| van Montfoort 2006 | Various ages, no criteria (mean age 33) | SET‐ 3.3+/‐1.8, DET‐ 3.3+/‐ 2.1 | First IVF cycle | Not included | yes | not mentioned | good |
| Vauthier‐Brouzes 1994 | ≤35 years | Not mentioned | First or previous successful cycle | Frozen cycles included | yes | not mentioned | good |
| ASSETT 2003 unpublished trial | Female age <35 if no previous ART pregnancy, <40 if | Not mentioned | First or previous successful cycle | Frozen cycles included | yes | not mentioned | At least four good quality |
| ECOSSE 2006 unpublished trial | ≤37 years | Not mentioned | first or second cycle of treatment | frozen cycles included | yes | not mentioned | 4 or more good quality embryos available at the time of embryo transfer |
Two studies did not provide details of participant characteristics (Komori 2004; Mostajeran 2006).
Interventions
All the studies included embryo transfer after fresh IVF or ICSI cycles and two studies included frozen cycles administered to one or both groups (Thurin 2004; Thurin 2005). Several other studies also administered frozen cycles during follow‐up but not as part of the randomised comparison (Vauthier‐Brouzes 1994; Martikainen 2001; ECOSSE 2006; Fernandez‐Sanchez 2012).
Interventions in the included studies were as follows:
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one fresh single embryo transfer (SET) plus one frozen embryo transfer (1FZET) in a natural or hormone‐stimulated cycle compared with one fresh cycle of double embryo transfer (DET) (Thurin 2004; Thurin 2005);
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two fresh cycles of SET compared with one fresh cycle of DET (Lukassen 2005);
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one fresh cycle of SET plus multiple cycles of frozen DET compared with one cycle of fresh DET plus multiple cycles of frozen DET (ECOSSE 2006)
-
one fresh cycle of SET compared with one fresh cycle of DET (Gerris 1999; Martikainen 2001; Gardner 2004; ASSETT 2003; van Montfoort 2006; Fernandez‐Sanchez 2012);
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one fresh cycle of DET compared with one fresh cycle of triple embryo transfer (TET) (Heijnen 2006);
-
fresh or frozen DET compared with fresh or frozen TET, multiple cycles (Komori 2004)
-
two fresh cycles of DET compared to two fresh cycles of TET (Heijnen 2006);
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three fresh cycles of DET compared to three fresh cycles of TET (Heijnen 2006);
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fresh DET compared with fresh TET where the number of cycles used was unclear (Mostajeran 2006);
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one fresh cycle of DET compared with one fresh cycle of four embryo transfer (FET) (Vauthier‐Brouzes 1994).
One study (Komori 2004) reported only per‐cycle data. There a large disparity between the number of women (169) and the number of cycles (212), and it was unclear how many women were included in each group. The data from this study were therefore unusable.
Four studies that randomised women to more than one embryo transfer cycle reported interim data after the first fresh cycle of SET versus DET (Thurin 2004; Thurin 2005; Lukassen 2005; ECOSSE 2006). In the case of ECOSSE 2006, these were the only data available, as the trial was stopped due to poor recruitment and data were only available for the first cycle (i.e. fresh DET versus fresh SET).
Protocols for ovarian stimulation, oocyte recovery and embryo transfer were clearly described in nine studies (Vauthier‐Brouzes 1994; Gerris 1999; Martikainen 2001; Thurin 2004; Lukassen 2005; Thurin 2005; Heijnen 2006; van Montfoort 2006; Fernandez‐Sanchez 2012). Good quality embryos were transferred in all studies, usually at cleavage stage. However, in four studies all or some women had embryos transferred at blastocyst rather than cleavage stage; this applied to a small number of women in two studies (Thurin 2004; Thurin 2005), half the women in one study (Fernandez‐Sanchez 2012) and all women in another study (Gardner 2004). The stage of embryo transfer was not mentioned in one study (Mostajeran 2006).
Natural progesterone was used for luteal phase support in most cases (Gerris 1999; Martikainen 2001; Gardner 2004; Thurin 2004; Lukassen 2005; Thurin 2005; Heijnen 2006; van Montfoort 2006; Fernandez‐Sanchez 2012). One study used both human chorionic gonadotropin (HCG) and natural progesterone for luteal phase support (Vauthier‐Brouzes 1994).
Outcomes
Primary outcomes
1. Live birth rate and cumulative live birth rate
Eleven studies reported live birth rate per couple (Vauthier‐Brouzes 1994; Gerris 1999; Martikainen 2001; ASSETT 2003; Thurin 2004; Lukassen 2005; Thurin 2005; ECOSSE 2006; Heijnen 2006; van Montfoort 2006; Fernandez‐Sanchez 2012). One reported 'take home baby' per cycle only (Komori 2004).
Five studies reported cumulative live birth rates (ASSETT 2003; Thurin 2004; Lukassen 2005; Thurin 2005; Heijnen 2006).
2. Multiple pregnancy rate per woman or couple
All but one study reported multiple pregnancy rate per couple. One reported multiple pregnancy per cycle only (Komori 2004).
Secondary outcomes
1. Clinical pregnancy rate
Ten studies reported pregnancy rate per couple (Vauthier‐Brouzes 1994; Gerris 1999; Martikainen 2001; Gardner 2004; Thurin 2004; Lukassen 2005; Heijnen 2006; Mostajeran 2006; van Montfoort 2006; Fernandez‐Sanchez 2012).
2. Miscarriage rate per woman
Three studies reported miscarriage rate (Martikainen 2001; Lukassen 2005; van Montfoort 2006).
Excluded studies
See Characteristics of excluded studies.
Fourteen studies were excluded from the review for the following reasons:
-
four studies were not randomised (Bowman 2004; van Montfoort 2005; Moustafa 2008; Guerif 2011);
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10 studies did not report a comparison of interest (Staessen 1993; Gardner 1998; Motta 1998 A & B; Livingstone 2001; Frattarelli 2003; Levitas 2004; Pantos 2004; Heijnen 2007; Elgindy 2011; Forman 2012).
Risk of bias in included studies
See Characteristics of included studies; Figure 1; Figure 2.
Allocation
Generation of random sequence
Ten studies were at low risk of bias related to random sequence generation as they used computer‐generated methods. Four studies did not describe their randomisation methods and were therefore at unclear risk of this bias.
Allocation concealment
Four studies were at low risk of bias related to allocation concealment. They used sealed opaque envelopes (ASSETT 2003) or remote allocation (ECOSSE 2006; Heijnen 2006; Fernandez‐Sanchez 2012). In the other ten studies a satisfactory method of allocation concealment was not described clearly enough or no information was given, and the risk of this bias was therefore rated as unclear.
Blinding
Five trials were rated as at low risk of bias related to blinding (ASSETT 2003; Thurin 2004; Thurin 2005; ECOSSE 2006; van Montfoort 2006) as neither the patient nor physician knew whether one embryo or two embryos had been transferred. Two studies were unblinded (Lukassen 2005; Fernandez‐Sanchez 2012) and the others did not mention blinding. These nine studies were rated as at unclear risk of bias as it was unclear whether lack of blinding would be likely to influence the outcomes of this review.
Incomplete outcome data
Ten studies were rated as at low risk of this bias as they included all randomised women in the analysis. Three studies were rated as at unclear risk of this bias because it was unclear how many women were included in the analysis (Gardner 2004; Komori 2004; Vauthier‐Brouzes 1994). One study (Mostajeran 2006) was rated as at high risk of this bias because it was unclear how many women were randomised: women non‐compliant with the drug regimen or who had ovarian hyperstimulation syndrome (numbers not stated) were excluded and three women with ectopic pregnancy were also excluded from the analysis.
Selective reporting
Eleven studies were deemed to be at low risk of this bias. Two studies (Gardner 2004; Mostajeran 2006) that did not report live birth and one study which only reported per cycle data (Komori 2004) were deemed to be at unclear risk of this bias.
Other potential sources of bias
Two studies were judged to be at low risk of other potential biases and 11 were at unclear risk. One study (Fernandez‐Sanchez 2012) gave women the option of changing the number of embryos transferred or the day of transfer if they were unhappy with the group to which they were randomised. A large number of participants (21%) chose to change, including 36% of women in the SET groups who changed to DET. Although the study was analysed by intention to treat, the results were deemed to be at high risk of bias due to the high level of non‐compliance and the fact that nearly all the changes were in the same direction.
Effects of interventions
See: Summary of findings for the main comparison Repeated single embryo transfer compared to double embryo transfer; Summary of findings 2 Single embryo transfer compared to double embryo transfer (in a single cycle)
The results below are formatted by type of comparison, as follows.
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Repeated single embryo transfer versus repeated multiple transfer
-
Repeated single embryo transfer versus mixed policies
-
Single versus multiple embryo transfer in a single cycle
-
Other fresh cycle comparisons.
1. Repeated single embryo transfer versus repeated multiple transfer.
No studies compared repeated single embryo transfer versus repeated multiple transfer.
2. Repeated single embryo embryo transfer versus mixed policies
Three studies, all of cleavage‐stage transfer, made this comparison (Thurin 2004; Lukassen 2005; Thurin 2005).
Specific interventions were as follows (with the number of cycles in brackets).
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Single embryo transfer (x 2) versus double embryo transfer (x 1) (SET (x2) versus DET (X1))(Lukassen 2005).
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Single embryo transfer (x 1) plus transfer of one frozen thawed embryo in a natural or hormone‐stimulated cycle versus double embryo transfer (x 1) (SET + 1 FZET versus DET (X1)) (Thurin 2004; Thurin 2005).
Primary outcomes
2.1 Cumulative live birth rate
When the three studies (Thurin 2004; Lukassen 2005; Thurin 2005) were pooled, the cumulative live birth rate after repeated single embryo transfer was not significantly different from the rate after one cycle of DET (OR 0.82, 95% CI 0.62, to 1.09, three studies, n=811, I2=0%). This suggests that for a woman with a 40% chance of live birth following a single cycle of DET, the chance following repeated SET would be between 31% and 44%.
2.1.1 SET + 1 FZET versus DET (x1)
Two studies reported cumulative live birth rates after SET followed by 1 FZET versus DET in a single cycle (Thurin 2004; Thurin 2005). The difference in cumulative live birth rate between SET + 1 FZET and DET was not statistically significant (OR 0.83, 95% CI 0.61 to 1.12, two studies, n = 703, I2 = 0%).
2.1.2 SET (x 2) versus DET (x1)
A single study compared cumulative live birth rate after two fresh cycles of SET versus a single fresh cycle of DET (Lukassen 2005). It did not find a significant difference between the two groups (OR 0.79, 95% CI 0.36 to 1.72, one study, n = 108).
See Analysis 2.1; Figure 4
Forest plot of comparison: 2 Repeated single versus mixed policies, outcome: 2.1 Cumulative live birth.
2.2 Multiple pregnancy rate
When the three studies (Thurin 2004; Lukassen 2005; Thurin 2005) were pooled, the multiple pregnancy rate after repeated single embryo transfer was significantly lower than after a single cycle of DET (OR 0.03, 95% CI 0.01 to 0.13, three studies, n=811, I2 = 23%). This suggests that for a woman with a 13% risk of multiple pregnancy following a single cycle of DET, the risk following repeated SET would be between 0% and 2%.
2.2.1 SET + 1 FZET versus DET (x 1)
Two studies reported multiple pregnancy rates after SET plus 1 FZET versus DET in a single cycle (Thurin 2004; Thurin 2005). There was a significantly lower multiple pregnancy rate in the SET group, with substantial heterogeneity (OR 0.03, 95% CI 0.01 to 0.14, two studies, n = 703, I2 = 60%). There was no obvious explanation for the heterogeneity.
2.2.2 SET (x 2) versus DET (x 1)
A single study compared the multiple pregnancy rate after two fresh cycles of SET versus a single fresh cycle of DET (Lukassen 2005) and did not find a significant difference between the two groups (OR 0.07, 95% CI 0.00 to 1.25, one study, n = 108).
See Analysis 2.2; Figure 5
Forest plot of comparison: 2 Repeated single versus mixed policies, outcome: 2.2 Multiple pregnancy.
Secondary outcomes
2.3 Clinical pregnancy rate
When two studies reporting this outcome (Lukassen 2005; Thurin 2004) were pooled, the clinical pregnancy rate after repeated single embryo transfer was not significantly different from the rate after one cycle of DET (OR 0.81, 95% CI 0.61 to 1.08, two studies, n=768, I2=0%)
2.3.1 SET + 1 FZET versus DET (x 1)
A single study reported the clinical pregnancy rate after SET followed by 1 FZET versus DET in a single cycle (Thurin 2004). No significant difference was found between the groups (OR 0.83 95% CI 0.61 to 1.12, one study, n = 661).
2.3.2 Fresh SET (x 2) versus DET (x 1)
A single study compared the clinical pregnancy rate after two fresh cycles of SET versus a single fresh cycle of DET (Lukassen 2005) and did not find a significant difference between the two groups (OR 0.71, 95% CI 0.33 to 1.53, one study, n= 107).
See Analysis 2.3
2.4 Miscarriage rate
A single study reported the miscarriage rate after two fresh cycles of SET versus a single fresh cycle of DET (Lukassen 2005). No significant difference was found between the two groups (OR 0.60, 95% CI 0.18 to 1.97, one study, n = 107).
See Analysis 2.4
3. Single versus multiple embryo transfer in a single cycle
Nine studies of cleavage‐stage transfer (Gerris 1999; Martikainen 2001; ASSETT 2003; Thurin 2004; Lukassen 2005; Thurin 2005; ECOSSE 2006; van Montfoort 2006; Fernandez‐Sanchez 2012) and two of blastocyst‐stage transfer (Gardner 2004; Fernandez‐Sanchez 2012) made this comparison. One reported both (Fernandez‐Sanchez 2012).
All compared one cycle of single versus one cycle of double embryo transfer (SET (x 1) versus DET (x 1)). As noted above, for four of these studies (Thurin 2004; Thurin 2005; Lukassen 2005; ECOSSE 2006) the data for this comparison derive from an interim analysis, as women in one or both arms were randomised to undergo further transfer cycles if the first cycle did not result in pregnancy.
Primary outcomes
3.1 Live birth rate
3.1.1 SET (x 1) versus DET (x 1)
Nine studies of cleavage‐stage transfer and one of blastocyst transfer reported this outcome. See Analysis 3.1; Figure 6
Forest plot of comparison: 3 Single versus multiple (in a single cycle), outcome: 3.1 Live birth.
When all studies were pooled, the live birth rate per woman was significantly lower in women who had SET than those who had DET (OR 0.48, 95% CI 0.39 to 0.60, nine studies, n = 1564, I2 = 0%). This suggests that for a woman with a 45% chance of live birth following a single cycle of DET, the chance following a single cycle of SET would be between 24% and 33%.
These findings applied in comparisons of cleavage‐stage transfer (OR 0.49, 95% CI 0.40 to 0.62, nine studies, n = 1464, I2 = 0%) and also in the single comparison of blastocyst transfer (OR 0.34, 95% CI 0.15 to 0.77, one study, n = 100).
A funnel plot for this outcome was not suggestive of publication bias. See Figure 7
Funnel plot of comparison: 3 Single versus multiple (in a single cycle), outcome: 3.1 Live birth.
3.2 Multiple pregnancy rate
3.2.1 SET (x 1) versus DET (x 1)
Nine studies of cleavage‐stage transfer and two of blastocyst transfer reported this outcome. See Analysis 3.2; Figure 8
Forest plot of comparison: 3 Single versus multiple (in a single cycle), outcome: 3.2 Multiple pregnancy.
When all studies were pooled, the multiple pregnancy rate per woman was significantly lower in those who had SET than those who had DET (OR 0.12, 95% CI 0.07 to 0.20, 10 studies, n = 1612, I2 = 45%). This suggests that for a woman with a 14% risk of multiple pregnancy following a single cycle of DET, the risk following a single cycle of SET would be between 1% and 3%
These findings applied in comparisons of cleavage‐stage transfer (OR 0.10, 95% CI 0.05 to 0.18, nine studies, n = 1464, I2 = 0%) and also in comparisons of blastocyst transfer (OR 0.25, 95% CI0.08 to 0.72, two studies, n = 148, I2 = 67%). Heterogeneity in these analyses appeared to derive from a study at high risk of bias (Fernandez‐Sanchez 2012). Treatment contamination (also known as ‘cross‐over’) occurred in a high proportion of cases in this study and would be expected to attenuate any treatment difference. I2 reduced to 0% when this study was excluded from the analyses, without materially affecting the conclusion.
In a sensitivity analysis restricted to studies which clearly reported methods of randomisation and allocation concealment and did not appear to be at high risk of bias, there were only three studies (ASSETT 2003; Lukassen 2005; ECOSSE 2006) with a total of 157 participants. Findings for live births for SET versus DET were no longer statistically significant (OR 0.68, 95% CI 0.35 to 1.34) but the findings for multiple pregnancy still significantly favoured SET (OR 0.13, 95% CI 0.02 to 0.74).
Secondary outcomes
3.3 Clinical pregnancy rate
3.3.1 SET (x 1) versus DET (x 1)
Six studies of cleavage‐stage transfer and two of blastocyst transfer reported this outcome. See Analysis 3.3
When all studies were pooled, the clinical pregnancy rate per woman was significantly lower in those who had SET than those who had DET (OR 0.46, 95% CI 0.37 to 0.57, seven studies, n = 1521, I2 = 0%).
These findings applied in comparisons of cleavage‐stage transfer (OR 0.46, 95% CI 0.37 to 0.57, six studies, n = 1357, I2 = 0%) and also in comparisons of blastocyst transfer (OR 0.37, 95% CI 0.18 to 0.76, two studies, n = 148, I2 = 0%).
Miscarriage rate
Three studies of cleavage‐stage transfer reported this outcome (Martikainen 2001; Thurin 2004; van Montfoort 2006). No significant difference was found between the two groups (OR 0.85, 95% CI 0.54 to 1.34, three studies, n = 1113, I2 = 61%), see Analysis 3.4
4. Other fresh cycle comparisons
Three studies tested other fresh cycle comparisons. Two were of cleavage‐stage transfer (Vauthier‐Brouzes 1994; Heijnen 2006). The day of transfer of the third study (Mostajeran 2006) was not reported. Specific interventions were as follows (with the number of cycles in brackets):
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DET (x 1) versus triple embryo transfer (TET) (x 1) (Heijnen 2006; Mostajeran 2006);
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DET (x 1) versus four embryo transfer (FET) (x 1) (Vauthier‐Brouzes 1994);
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DET (x 2) versus TET (x 2) (Heijnen 2006);
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DET (x 3) versus TET (x 3) (Heijnen 2006).
Primary outcomes
4.1 Live birth rate or cumulative live birth rate across single or repeated IVF cycles
4.1.1 DET (x 1) versus TET (x 1)
No significant difference was found between the groups in the live birth rate (OR 0.40, 95% CI 0.09 to 1.85, one study, n = 45) (Heijnen 2006).
4.1.2 DET (x 1) versus FET (x 1)
No significant difference was found between the groups in the live birth rate (OR 0.35, 95% CI 0.11 to 1.05, one study, n = 56) (Vauthier‐Brouzes 1994).
4.1.3 DET (x 2) versus TET (x 2)
No significant difference was found between the groups in the cumulative live birth rate after two cycles of SET versus two cycles of TET (OR 0.77, 95% CI 0.22 to 2.65, one study, n = 45) (Heijnen 2006).
4.1.4 DET (x 3) versus TET (x 3)
No significant difference was found between the groups in the cumulative live birth rate after three cycles of SET versus three cycles of TET (OR 0.77, 95% CI 0.24 to 2.52, one study, n = 45) (Heijnen 2006).
See Analysis 4.1.
4.2 Multiple pregnancy rate
4.2.1 DET (x 1) versus TET (x 1)
There was a significantly lower multiple pregnancy rate in the DET group than in the TET group (OR 0.36, 95% CI 0.13 to 0.99, two studies, n = 343) (Heijnen 2006; Mostajeran 2006).
4.2.2 DET (x 1) versus FET (x 1)
No significant difference was found between the groups in the multiple pregnancy rate (OR 0.44, 95% CI 0.10 to 1.97, one study, n = 56) (Vauthier‐Brouzes 1994).
See analysis Analysis 4.3.
Secondary outcomes
4.3 Clinical pregnancy rate
4.3.1 DET (x 1) versus TET (x 1)
There was no significant difference between the groups in the clinical pregnancy rate (OR 0.67, 95% CI 0.42 to 1.08, one study, n = 343) (Heijnen 2006).
4.3.2 DET versus FET
No significant difference was found between the groups in the clinical pregnancy rate (OR 0.56, 95% CI 0.19 to 1.62, one study, n = 56) (Vauthier‐Brouzes 1994).
4.4 Miscarriage rate
No studies reported this outcome.
5. Other fresh or frozen cycle comparisons
One study (Komori 2004) of cleavage‐stage transfer compared DET versus TET among 169 participants. A total of 106 cycles of fresh or frozen embryos were apparently administered in each group, but study reporting was unclear and, moreover, outcomes were reported per cycle rather than per woman. Attempts to contact the authors were unsuccessful. Study findings were reported descriptively below.
Primary outcomes
5.1 Cumulative live birth rate
5.1.1 DET versus TET, apparently using fresh or frozen embryos for multiple cycles
No significant difference was found between the groups for this outcome using per cycle data (30 versus 26 live births resulting from 106 cycles in each group) (Komori 2004).
5.2 Multiple pregnancy rate
5.2.1 DET versus TET, apparently using fresh or frozen embryos for multiple cycles
There was a significantly lower incidence of multiple births per pregnancy in the DET group (6/40 pregnancies versus 14/29 pregnancies) (Komori 2004).
Secondary outcomes
5.3 Clinical pregnancy rate
5.3.1 DET versus TET, apparently using fresh or frozen embryos for multiple cycles
No significant difference was found between the groups for this outcome using per cycle data (40 versus 29 pregnancies resulting from 106 cycles in each group) (Komori 2004).
5.4 Miscarriage rate
This outcome was not reported.
Subgroup and sensitivity analyses
We did not perform our planned subgroup analyses to assess the efficacy of embryo replacement protocols in participant groups with differing prognostic characteristics because most studies did not identify such subgroups.
There were insufficient studies which clearly reported methods of randomisation and allocation concealment to conduct sensitivity analyses by study quality, other than for analysis 3.1. The overall findings did not materially change with the use of a random‐effects model rather than a fixed‐effect model or with use of risk ratios rather than odds ratios.
Discussion
Summary of main results
Our findings indicate, as one would expect, that live birth and pregnancy rates following single embryo transfer (SET) are lower than those following double embryo transfer (DET) in a single fresh IVF cycle but that the risk of multiple pregnancy is much higher in the DET group. However, pooling of three studies of cleavage‐stage transfer found no evidence of a significant difference in the cumulative live birth rate when a single cycle of DET was compared with repeated SET (either SET followed by transfer of a single frozen embryo in a natural or hormone‐stimulated cycle (Thurin 2004; Thurin 2005), or two fresh cycles of SET (Lukassen 2005)). Confidence intervals for this finding were wide, and suggested that for a woman with a 42% chance of live birth following a single cycle of DET, the chance following repeated SET would be between 31% and 44%.
Thus, although DET achieves higher live birth rates per fresh cycle, the evidence suggests that the difference in effectiveness may be substantially offset when elective SET is followed by a further single fresh or frozen cycle, at least among women with a good prognosis.
Eleven studies compared one fresh cycle of SET versus one fresh cycle of DET. The live birth rate was 60% higher in the DET group but the risk of multiple pregnancy was eight times as high. One of this group of studies included a high proportion of women who chose not to comply with their randomised treatment, and inclusion of this study was associated with substantial heterogeneity for the outcome of multiple pregnancy. Otherwise there was little evidence of statistical heterogeneity in the review, suggesting that clinical differences between studies had little effect on overall findings.
Three studies of cleavage‐stage transfer tested fresh cycle comparisons of DET versus transfer of three or four embryos. Live birth rates did not differ significantly, but there was a significantly lower multiple pregnancy rate in the DET group than in the three embryo transfer (TET) group.
Overall completeness and applicability of evidence
No studies compared repeated single versus repeated multiple embryo transfer within the same IVF cycle. This comparison was planned in one study (ECOSSE 2006) but the study was closed due to poor enrolment, with only 23 participants. This comparison would be a useful way to structure future trials in order to determine the safety and effectiveness of different embryo transfer policies, given that a number of embryos have been produced. Policy in this context means the strategy for using up the available embryos until success is achieved or the supply of embryos is exhausted. A comparison of repeated multiple versus repeated single embryo transfer would address the policy question by determining ‘cumulative’ success rates.
The vast majority of participants in the included studies had a good prognosis (aged under 36 years and with sufficient good quality embryos). Only two small studies (Thurin 2005; Heijnen 2006) focused on older women. As one of the studies (Gardner 2004) noted, there was a strong potential for self‐selection bias, as only a small proportion of eligible women volunteered for the trial, probably due to the belief that single ET could result in lower pregnancy rates and that twin pregnancy is a desirable outcome: they commented that most volunteers were younger women. Future studies should include older women and those with previously failed IVF cycles or lack of good quality embryos
Per cycle, DET appears to be more expensive than SET (Tiitinen 2001; Gerris 2004; Thurin 2006; Chambers 2007; Fiddelers 2007). The higher cost is mainly due to the increased rate of multiple births and premature births in the DET group, and fewer pregnancies in the SET group. Long term costs related to multiple births and prematurity in the DET group have not yet been adequately assessed. However the additional costs of cryopreservation with SET + 1 FZET have not been evaluated. In order to implement a policy of multiple single embryo transfers per woman, providers require either an efficient cryopreservation service or the ability to provide multiple fresh IVF cycles. The former is likely to be a safer and less invasive option for the women concerned.
Only two studies (Gardner 2004; Fernandez‐Sanchez 2012) specifically addressed blastocyst transfer.
Quality of the evidence
Many of the included studies were small, with half enrolling fewer than 60 participants. There was considerable clinical heterogeneity between the studies but little evidence of statistical heterogeneity for most analyses. The methodological quality of the studies was mixed. See Figure 2. Confidence intervals were wide for some analyses, and GRADEPro ratings for the primary outcomes ranged from high (for comparisons of DET versus SET in a single cycle) to low or very low (for comparisons of DET versus repeated SET). See Summary of findings table 3; Summary of findings table 4).
Potential biases in the review process
One of the review authors is primary investigator of one of the included studies (ECOSSE 2006).
Our comparison of one cycle of fresh SET versus one cycle of DET (Analysis 6.1) includes data from studies for which this was an interim analysis. This may be a potential source of bias, associated with placebo effects relating to participant anxiety. A post‐hoc sensitivity analysis excluding these studies did not materially influence the live birth rate in this analysis.
We are unaware of any other potential biases in the review process.
Agreements and disagreements with other studies or reviews
Other studies and reviews are broadly in agreement with the current review.
A project commissioned by the UK National Institutes of Health Research Health Technology Assessment Programme (Roberts 2011) used statistical modelling, analysis of registry and cohort data, and exploration of consumer perspectives to explore options for increasing SET and reducing the incidence of multiple births. The analysis concluded that couples have approximately one‐third less chance of a live birth if they have one fresh cycle of SET rather than DET, but that use of repeat cycles using cryopreservation might compensate for the lost potential in each individual transfer while reducing the likelihood of multiple births. However, the authors recognised that a policy of repeat SET (with use of cryopreserved eggs) would involve challenges including appropriate patient selection, optimisation of freezing techniques, and the emotional, financial and physical burden associated with additional treatment cycles.
Recent systematic reviews (Gelbaya 2010; McLernon 2010) and a report from the American Society for Reproductive Medicine (ASRM 2012) have reached similar conclusions.
A large Dutch cohort study is currently in progress, which aims to assess the long term costs and health outcomes of IVF singleton and twin children and the long term cost‐effectiveness of SET versus DET strategies. Outcomes will be reported at one year, five years and 18‐year follow‐up (van Heesch 2010).
Methodological quality graph: review authors' judgements about each methodological quality item presented as percentages across all included studies.
Methodological quality summary: review authors' judgements about each methodological quality item for each included study.
Forest plot of comparison: 2 Repeated single versus mixed policies, outcome: 2.1 Cumulative live birth.
Forest plot of comparison: 2 Repeated single versus mixed policies, outcome: 2.2 Multiple pregnancy.
Forest plot of comparison: 3 Single versus multiple (in a single cycle), outcome: 3.1 Live birth.
Funnel plot of comparison: 3 Single versus multiple (in a single cycle), outcome: 3.1 Live birth.
Forest plot of comparison: 3 Single versus multiple (in a single cycle), outcome: 3.2 Multiple pregnancy.
Comparison 2 Repeated single versus mixed policies, Outcome 1 Cumulative live birth.
Comparison 2 Repeated single versus mixed policies, Outcome 2 Multiple pregnancy.
Comparison 2 Repeated single versus mixed policies, Outcome 3 Clinical pregnancy rate.
Comparison 2 Repeated single versus mixed policies, Outcome 4 Miscarriage.
Comparison 3 Single versus multiple (in a single cycle), Outcome 1 Live birth.
Comparison 3 Single versus multiple (in a single cycle), Outcome 2 Multiple pregnancy.
Comparison 3 Single versus multiple (in a single cycle), Outcome 3 Clinical pregnancy rate.
Comparison 3 Single versus multiple (in a single cycle), Outcome 4 Miscarriage.
Comparison 4 Other fresh cycle comparisons, Outcome 1 Live or cumulative live birth.
Comparison 4 Other fresh cycle comparisons, Outcome 2 Multiple pregnancy.
Comparison 4 Other fresh cycle comparisons, Outcome 3 Clinical pregnancy.
| Repeated single compared to mixed policies for transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Population: women having embryo transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Outcomes | Illustrative comparative risks* (95% CI) | Relative effect | No of Participants | Quality of the evidence | Comments | |
| Assumed risk | Corresponding risk | |||||
| Double ET | Repeated single ET | |||||
| Cumulative live birth Repeated single embryo transfer versus double embryo transfer | 420 per 1000 | 373 per 1000 | OR 0.82 (0.62 to 1.09 ) | 811 | ⊕⊕⊝⊝ | |
| Cumulative live birth ‐ Single embryo transfer plus one cycle of frozen embryo transfer versus one cycle of double embryo transfer | 422 per 1000 | 377 per 1000 | OR 0.83 (0.61 to 1.12 ) | 703 | ⊕⊕⊝⊝ | |
| Cumulative live birth ‐ Two cycles of single embryo transfer SET (x2) versus one cycle of double embryo transfer | 407 per 1000 | 352 per 1000 | OR 0.79 (0.36 to 1.72 ) | 108 | ⊕⊝⊝⊝ | |
| Multiple pregnancy Repeated single embryo transfer versus double embryo transfer | 133 per 1000 | 5 per 1000 | OR 0.03 (0.01 to 0.13 ) | 811 | ⊕⊕⊝⊝ | |
| Multiple pregnancy Single embryo transfer plus one cycle of frozen embryo transfer versus one cycle of double embryo transfer | 136 per 1000 | 5 per 1000 | OR 0.03 (0.01 to 0.14 ) | 703 | ⊕⊕⊝⊝ | |
| Multiple pregnancy Two cycles of single embryo transfer SET (x2) versus one cycle of double embryo transfer | 111 per 1000 | 9 per 1000 | OR 0.07 (0.00 to 1.25 ) | 108 | ⊕⊝⊝⊝ | |
| *The basis for the assumed risk is the median control group risk across studies. The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). | ||||||
| GRADE Working Group grades of evidence | ||||||
| 1Methods of allocation concealment not described in enough detail 2Wide confidence intervals 3One small study | ||||||
| Single compared to multiple embryo transfer (in a single cycle) following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Population: women having embryo transfer following in vitro fertilisation or intra‐cytoplasmic sperm injection | ||||||
| Outcomes | Illustrative comparative risks* (95% CI) | Relative effect | No of Participants | Quality of the evidence | Comments | |
| Assumed risk | Corresponding risk | |||||
| Multiple | Single | |||||
| Multiple pregnancy | 144 per 1000 | 20 per 1000 | OR 0.12 (0.07 to 0.20 ) | 1612 | ⊕⊕⊕⊕ | |
| Live birth | 450 per 1000 | 282 per 1000 | OR 0.48 (0.39 to 0.60 ) | 1564 | ⊕⊕⊕⊕ | |
| *The basis for the assumed risk is the median control group risk across studies). The corresponding risk (and its 95% confidence interval) is based on the assumed risk in the comparison group and the relative effect of the intervention (and its 95% CI). | ||||||
| GRADE Working Group grades of evidence | ||||||
| 136% of women noncompliant with treatment allocation in one study. However, no statistical heterogeneity detected (I2=0%) 2Moderate heterogeneity attributable to 36% of women oncompliant with treatment allocation in one study (I2=45%) | ||||||
| Study author and year | Age Eligibility criteria (mean participant age, where stated) | Duration of infertility | Previous failed cycle | Frozen cycles | Prim/Sec infertility | FSH | Quality of embryo |
| Fernandez‐Sanchez 2012 | Under 38 years (mean age 33) | Mean 2.6 to 3.2 years | First IVF/ICSI cycle. | Frozen cycles included | Not stated | Not stated | good |
| Gerris 1999 | less than 34 years | Average duration of infertility 3.5 years. | First IVF/ICSI cycle. | Not included | unclear | not mentioned | good |
| Heijnen 2007 | 38‐45 years (mean age 41) | Average duration of infertility in DET group was 3.7(+/‐ 2.5) and in TET group was 3.2(+/‐ 2.4) | First cycle and previous successful cycle | Not included | yes | not mentioned | good |
| Komori 2004 | Not stated | Not stated | Not stated | Not stated | Not stated | Not stated | Good |
| Lukassen 2005 | <35 years (mean age 30‐31) | not stated | First IVF/ICSI cycle or after previous successful cycle . | Not included | yes | FSH < 10IU/L. | good |
| Martikainen 2001 | various, no age criteria, ranged between 22‐40 years (mean age 31) | not stated | women who had / not had more than one previous failed treatment. | Frozen cycles included | yes, but not mentioned | not mentioned | good |
| Mostajeran 2006 | Not stated | Not stated | Not stated | Not stated | Not stated | Not stated | Good |
| Thurin 2004 | <36 years (mean age 31) | 0‐12 years | First or second IVF cycle | Frozen cycles included | yes | not mentioned | good, blastocysts included |
| Thurin 2005 Unpublished trial, pilot study, part of a thesis | ≥36 years | 0‐12 years | First or second IVF/ICSI cycle | Frozen cycles included | yes | not mentioned | At |
| van Montfoort 2006 | Various ages, no criteria (mean age 33) | SET‐ 3.3+/‐1.8, DET‐ 3.3+/‐ 2.1 | First IVF cycle | Not included | yes | not mentioned | good |
| Vauthier‐Brouzes 1994 | ≤35 years | Not mentioned | First or previous successful cycle | Frozen cycles included | yes | not mentioned | good |
| ASSETT 2003 unpublished trial | Female age <35 if no previous ART pregnancy, <40 if | Not mentioned | First or previous successful cycle | Frozen cycles included | yes | not mentioned | At least four good quality |
| ECOSSE 2006 unpublished trial | ≤37 years | Not mentioned | first or second cycle of treatment | frozen cycles included | yes | not mentioned | 4 or more good quality embryos available at the time of embryo transfer |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Cumulative live birth Show forest plot | 3 | 811 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.82 [0.62, 1.09] |
| 1.1 SET + 1 FZET versus DET (x1) (cleavage stage) | 2 | 703 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.83 [0.61, 1.12] |
| 1.2 SET (x2) versus DET (x1) (cleavage stage) | 1 | 108 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.79 [0.36, 1.72] |
| 2 Multiple pregnancy Show forest plot | 3 | 811 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.03 [0.01, 0.13] |
| 2.1 SET + 1 FZET versus DET (x1) (cleavage stage) | 2 | 703 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.03 [0.01, 0.14] |
| 2.2 SET (x2) versus DET (x1) (cleavage stage) | 1 | 108 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.07 [0.00, 1.25] |
| 3 Clinical pregnancy rate Show forest plot | 2 | 768 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.81 [0.61, 1.08] |
| 3.1 SET + 1 FZET versus DET (x1) (cleavage stage) | 1 | 661 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.83 [0.61, 1.12] |
| 3.2 SET (x2) versus DET (x1) (cleavage stage) | 1 | 107 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.71 [0.33, 1.53] |
| 4 Miscarriage Show forest plot | 1 | Odds Ratio (M‐H, Fixed, 95% CI) | Subtotals only | |
| 4.1 SET (x2) versus DET (x1) (cleavage stage) | 1 | 107 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.60 [0.18, 1.97] |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Live birth Show forest plot | 9 | 1564 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.48 [0.39, 0.60] |
| 1.1 SET (x1) versus DET (x1) (cleavage stage) | 9 | 1464 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.49 [0.40, 0.62] |
| 1.2 SET (x1) versus DET (x1) (blastocyst stage) | 1 | 100 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.34 [0.15, 0.77] |
| 2 Multiple pregnancy Show forest plot | 10 | 1612 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.12 [0.07, 0.20] |
| 2.1 SET (x1) versus DET (x1) (cleavage stage) | 9 | 1464 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.10 [0.05, 0.18] |
| 2.2 SET (x1) versus DET (x1) (blastocyst stage) | 2 | 148 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.25 [0.08, 0.72] |
| 3 Clinical pregnancy rate Show forest plot | 7 | 1521 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.46 [0.37, 0.57] |
| 3.1 SET (x1) versus DET (x1) (cleavage stage) | 6 | 1373 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.47 [0.37, 0.59] |
| 3.2 SET (x1) versus DET (x1) (blastocyst stage) | 2 | 148 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.37 [0.18, 0.76] |
| 4 Miscarriage Show forest plot | 3 | Odds Ratio (M‐H, Fixed, 95% CI) | Subtotals only | |
| 4.1 SET (x1) versus DET (x1) (cleavage stage) | 3 | 1113 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.85 [0.54, 1.34] |
| Outcome or subgroup title | No. of studies | No. of participants | Statistical method | Effect size |
| 1 Live or cumulative live birth Show forest plot | 2 | Odds Ratio (M‐H, Fixed, 95% CI) | Subtotals only | |
| 1.1 DET (x1) versus TET (x1) | 1 | 45 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.4 [0.09, 1.85] |
| 1.2 DET (x1) versus FET (x1) | 1 | 56 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.35 [0.11, 1.05] |
| 1.3 DET (x2) versus TET (x2) | 1 | 45 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.77 [0.22, 2.65] |
| 1.4 DET (x3) versus TET (x3) | 1 | 45 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.77 [0.24, 2.52] |
| 2 Multiple pregnancy Show forest plot | 3 | Odds Ratio (M‐H, Fixed, 95% CI) | Subtotals only | |
| 2.1 DET versus TET (cleavage stage) | 2 | 343 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.36 [0.13, 0.99] |
| 2.2 DET versus FET (cleavage stage) | 1 | 56 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.44 [0.10, 1.97] |
| 3 Clinical pregnancy Show forest plot | 3 | Odds Ratio (M‐H, Fixed, 95% CI) | Subtotals only | |
| 3.1 DET (x1) versus TET (x1) (cleavage stage) | 2 | 343 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.67 [0.42, 1.08] |
| 3.2 DET (x1) versus FET (x1) (cleavage stage) | 1 | 56 | Odds Ratio (M‐H, Fixed, 95% CI) | 0.56 [0.19, 1.62] |
