Cumulative live birth rates according to the number of oocytes retrieved following the “freeze-all” strategy

After the advent of controlled ovarian stimulation (COS), multi-follicular stimulation cycles replaced natural mono-follicular cycles in the clinical practice of assisted reproductive technology (ART) [1]. A conventional in vitro fertilization (IVF) procedure starts with COS and ends with the transfer of the best available embryo [1], while all surplus embryos of adequate quality are cryopreserved for later use [2]. However, under some circumstances, a fresh embryo transfer cannot be performed, and the entire cohort of viable embryos is cryopreserved [1]. This is known as the “freeze-all” strategy, in which all embryos are cryopreserved for future frozen-thawed embryo transfer, and this approach is increasingly being favoured [3]. There are many indications for the “freeze-all” strategy: ovarian hyperstimulation syndrome (OHSS), the asynchrony of embryo and endometrial receptivity, preimplantation genetic diagnosis (PGD), preimplantation genetic screening (PGS) [3], fertility preservation and premature progestin elevation [4]. OHSS, an iatrogenic complication of COS accompanied by a higher number of retrieved oocytes, accounts for a substantial proportion of these indications. A strategy comprising the use of a GnRH antagonist for pituitary downregulation, a GnRH agonist for ovulation triggering, and vitrification of all embryos can achieve an OHSS-free status for high responders [3]. In recent years, randomized controlled trials and observational studies have shown that better in vitro fertilization pregnancy outcomes and lower rates of obstetric and perinatal morbidity are achieved following the first frozen-thawed embryo transfer of the “freeze-all” strategy than the fresh embryo transfer [5‐7]. Furthermore, some studies have examined the association between the number of oocytes retrieved and the cumulative live birth rate (CLBR) after one fresh and several frozen-thawed embryo transfers following one round of oocyte retrieval, and all of these studies found a positive association [8, 9]. However, almost no study has examined the association between the number of oocytes retrieved and the CLBR when the “freeze-all” strategy is used. Therefore, we carried out the current study to investigate the relationship between the number of oocytes retrieved and the CLBR for the “freeze-all” strategy.

In the current study, the median number of oocytes retrieved was 20 (interquartile range 16–24), and the distribution of aspirated oocytes is shown in Fig. 2. The median age of the women was 28 (interquartile range 25–30) years. The overall CLBR was 91.26%, and the cumulative clinical pregnancy rate (CCPR) was 93.68%.

The present study shows that among patients younger than 35 years old, the number of oocytes retrieved was an independent positive predictor of the CLBR in patients using the “freeze-all” strategy. The chance of having a live birth was 91.26% after a completed “freeze-all” cycle.

The impact of a higher number of oocytes retrieved after ovarian stimulation has been investigated in many prior studies. Advocates of mild stimulation suggest that obtaining fewer oocytes could achieve a lower embryo aneuploidy rate and a better implantation rate [10, 11]. In contrast, other studies have reported that the numbers of usable blastocysts and euploid embryos increased with the number of oocytes retrieved [12]. These findings may explain the positive association between the number of oocytes retrieved and the CLBR. Consistent with these findings, in our study, the fertilization rate was lower, but the CLBR and CCPR were higher in groups 3, 4, and 5 than in groups 1 and 2. Our findings also showed that groups 3, 4, and 5 had higher basal FSH, basal AFC and peak E2 levels on HCG day than were found in groups 1 and 2. Similar results were found by Fatemi et al. [9]. In our study, although the included patients either showed a tendency to have OHSS or had OHSS, we eliminated the influence of this factor by administering hydroxyethyl starch and albumin as a plasma expander for several days and cancelling the fresh embryo transfer by freezing all usable embryos. These measures can bypass the detrimental effects of supraphysiological oestradiol levels and improve endometrial receptivity in a deferred frozen-thawed embryo transfer.

To date, studies investigating the association between the number of oocytes retrieved and the CLBR for the “freeze-all” strategy were scarce. However, we found ten studies to date that evaluated the association between the number of oocytes retrieved and CLBR following a conventional in vitro fertilization strategy, which consists of a single fresh and all frozen-thawed embryo transfers after one aspiration, and all found a positive correlation between the number of oocytes retrieved and the CLBR [8, 9, 12‐15]. Apart from showing a positive correlation, some of these ten studies determined what oocyte number was ideal for the optimal CLBR. Magnusson et al. suggested that retrieving 18 oocytes would be optimal for cumulative live birth in patients of all ages [16]. Without mentioning age, Chen et al. from China indicated that when the oocyte number was more than 10 for PCOS patients, the CLBR did not increase [17]. A paper published in Human Reproduction showed that the ideal number of retrieved oocytes was approximately 25 for the optimal CLBR among women aged 18–35 years, while retrieving more than 30 oocytes was optimal in women aged 36–44 years, and retrieving 9 oocytes was ideal for women older than 45 years [18]. Although there was a great deal of heterogeneity in parameters such as sample size, patient age, study design, stimulation protocols et al. among these ten studies, they convinced us that there is a strong positive correlation between the number of oocytes retrieved and the CLBR in conventional in vitro fertilization strategies. However, none of these ten studies analysed the CLBR for “freeze-all” strategy. On the basis of the ten previous studies, our study further enriches the field and confirms that the same pattern exists for the “freeze-all” strategy: unadjusted results showed that the CLBR significantly increased as the number of oocytes retrieved increased and reached as high as 93.8% when 21–30 oocytes were aspirated and then did not have a significant increase. Multivariate logistic regression revealed that the number of oocytes retrieved remained an independent predictive factor for CLBR. Our findings are supported by a few studies investigating the “freeze-all” strategy. A prospective cohort observational study showed that in the “freeze-all” cycle, a higher number of frozen blastocysts was associated with a higher CLBR in women younger than 40 years old with a normal/high response and that the highest CLBR, 88%, occurred when more than 7 blastocysts were frozen [19]. However, that study was limited by its small sample size of 254 patients. Another retrospective cohort study showed that the CLBR improved as the number of oocytes yielded increased up to 25 oocytes when using the “freeze-all” strategy [20]. However, the author did not clarity the reason why all the embryos were frozen in his study. Freezing all embryos due to fertility preservation and due to OHSS are two completely different situations. Our study eliminated this heterogeneity by including the OHSS-only patients and showed that the upper limit for the number of oocytes retrieved should be no higher than 30 when using the “freeze-all” strategy. Our study also shows that the fertilization rate positively affected while the gonadotropin dosage negatively affected the CLBR. These findings are consistent with previous studies [13, 21].

The dropout rate in our study was 11.5%. Among the patients who discontinued, we found that as the number of oocytes increased, the attempted number of embryo transfers increased, the ratio of the number of embryos used to the number of oocytes yielded increased, and the ratio of the number of oocytes left to the number of oocytes yielded decreased. These findings were consistent with a previous study in which patients in whom fewer oocytes were retrieved were more likely to discontinue further treatment due to the couple’s perception of their poor treatment prognosis and the psychological burden [22].

Given that the patients in our study were younger than 35 years old, we cannot provide any guidance for patients of advanced age. Although we tried our best to reduce selection and confounding biases, there are still inherent biases related to the retrospective design of this study; these include changes in clinical practice that occurred during the long study period. Finally, the number of patients in whom more than 40 oocytes were retrieved was small (N = 36). However, this still represents the real number of oocytes retrieved, as a yield of more than 40 oocytes is rare in the real world. Despite the above limitations, we believe that our study has significant implications for clinicians and patients who must make informed decisions regarding the “freeze-all” strategy. However, we do not recommend that an extreme oocyte yield should be a goal given the risks of ovarian torsion, venous thrombo-embolism and arterial thrombosis, particularly because achieving an oocyte retrieval number higher than 30 is pointless when using the “freeze-all” strategy. This may be because younger women should have a sufficient number of euploid embryos to provide multiple possibilities for achieving a live birth, and the surplus embryos would therefore be wasted in these individuals [15].

In conclusion, when using the “freeze-all” strategy, the number of oocytes retrieved is a positive predictor of the CLBR among women less than 35 years old. Given that the CLBR significantly increased with the number of oocytes retrieved, reaching as high as 93.82% when 21–30 oocytes were aspirated, the ovarian stimulation should be rational, and the upper limit of the oocyte yield should be no more than 30 to avoid severe iatrogenic complications.