Birth weight centiles by gestational age for twins born in south India

Rate of twin births is rising due to the increased use of assisted reproductive technologies in the recent years [1]. Birth weight of twins are considerably lower than singletons and associated with higher risk for adverse perinatal and infant outcomes [2, 3]. Birth weight centiles by gestational age is often used as a health indicator and to understand the natural extent of variation in birth weight. There have been many studies of twin birth weight centiles worldwide [4‐7], but are of limited use in developing settings like India owing to the use of non-representative populations. In India, information on twins is quite limited, and most previous studies on centile curves focused only on singleton births [8‐10]. Currently, the clinical practice is that centiles curves for singleton births are used as an estimate to evaluate twin births. However, recent studies suggest use of singleton centile curves on twins is not appropriate as twins experience different growth trajectories than singletons [11]. Moreover, several studies have recommended the development of twin specific centile curves to evaluate twin births [12‐15].

Twin births are further complicated by placental chorionicity. Monochorionic twins present a two to three times higher risk for adverse outcomes than dichorionic twins [16], with birth weights of monochorionic twins lower than those of dichorionic twins over the gestational ages. Further, monochorionic placentation increases the risk of serious pregnancy complications (such as twin-to-twin transfusion syndrome), congenital anomalies, growth restriction, and perinatal death [17].

Past studies have indicated that placental chorionicity should be taken into consideration in assessment of twin births [18]. The lack of centile curves specific to twins could be a limiting factor in understanding the distribution of birth weight and further evaluation of twin births. Therefore, we carried out the present study to construct gestational age specific birth weight centile curves for twins born in South India, stratified by placental chorionicity (monochorionic and dichorionic placentation).

During the study period, a total of 1673 multiple pregnancies were delivered. Of which, the following were removed sequentially from further analysis (triplets = 39; fetal deaths of one or more foetuses = 141; missing data on chorionicity = 170; and missing data on birth weight and/or gestational age = 19). Thus complete data were available from 1304 twin gestations for analyses. The mean maternal age of mothers included in the sample was 25.2 years (SD = 4.3) and 46 % of mothers were primigravid. A total of 88 (6.7 %) mothers were conceived using some form of assisted reproductive technologies, while approximately (64) 5 % of mothers experienced gestational diabetes and (224) 17 % had preeclampsia. Eighty two percent of the women in our sample were Hindus, 11 % were Muslims and 7 % were Christians. About 8 % (106) of mothers were illiterates. There were 457 (35 %) monochorionic pregnancies.

The median gestational age at birth was 36 weeks (IQR 34–38 weeks). There was a 4.1 % increase in adjusted (for gestational age) mean birth weight from 2050 g in 1991 to 2135 in 2005. Dichorionic twins were heavier than monochorionic twins with an adjusted (for gestational age) mean of 2138 g compared with 2, 054 g respectively. The mean birth weight discordance was 13.1 % (SD = 10.3 %, median = 11.1 %). Considering a threshold of 18 % [18], birth weight discordance was identified in 360 out of 1304 pregnancies (27.6 %).

Examining the 50th centile, the weekly increase in birth weight flattens by 38 weeks of gestation and thereafter gain in the median birth weight was negligible (Fig. 1). To assess the validity of the fitted model, the expected percentage of observed birth weights below each centile was compared with observed percentage across gestational ages. About, 9 % fell below the 10th centile, 80.4 % between 10th and 90th centile and 9.9 % above the 90th centile. Further, the fit of the curves estimated from the statistical models were confirmed by overlaying the empirical centiles on top of the smoothed centiles (Fig. 1).

Figure 2 present birth weight centile curves for twins according to placental chorionicity. Monochorionic twins were consistently smaller than dichorionic twins after 27 weeks of gestation, with a fall-off across centiles of birthweight (Tables 1 and 2).

In this study, we constructed new birth weight centile curves for twins born in South India. We have presented centile curves by chorionic placentation to facilitate consideration of chorionicity in the assessment of twin births.

The overall pattern of change in birthweight over gestational age was characterized by a rapid change in weight till 38 weeks and reduction in change then onwards. Given that it is increasingly possible to determine chorionicity prenatally, it is important to consider placental chorionicity in the assessment of growth in twins. Our comparison of centile curves by chorionicity showed that birth weights of monochorionic twins were lower than dichorionic twins in gestational ages between 28 and 42 weeks. This could be explained by the increased demands with advancing gestational age in monochorionic twins which share a common placenta and this heightened demand may not be met as adequately as in dichorionic twins- leading to the difference between two groups.

Previous studies on distribution of birth weights in India have mainly been based on singleton births. Birth weights from our study were consistently lower than those of singletons [8]; the differences were approximately 500 g between gestational ages 32 and 42 weeks. This difference was similar to that seen in other published studies on the birth weight centile curves for twins [4, 6].

The data presented here is based on the largest sample size reported till date from India. However, in developing settings like India, it is considerably difficult to obtain precise obstetrical records on measurements at birth for a large number of twins, as there are not many population based twin registries. The new birth weight curves may provide useful evidence for better understanding the birth weight of twins born in South India. For instance, it could serve as a useful tool for clinicians to evaluate and assess the birth weight of newborn twins. Additionally, this new centile curves should be a useful for epidemiologic research on twins related to determination of geographic differences, temporal trends and etiologic determinants of distribution of birth weight.

One of some limitations of this study is that the data were drawn from a tertiary care hospital, and hence it may restrict the generalizability of our results. However, given that twin pregnancies are considered as high-risk and often referred to tertiary care hospitals, the problems related to generalizability might be less likely. Another limitation is the measurement of gestational age using dates of last menstrual period, which suffers from recall bias. We believe our estimates are likely to be improved with the use of early ultrasound to correct estimates of gestational age. Also, data on birthweight and placental chorionicity captured during the course of routine clinical care may not be as precise as measurements under more controlled research settings. Thus, for example, we were not able to ascertain the extent of intra or inter observer variability. Additionally, the number of infants in extreme gestational ages was not sufficiently larger to enable accurate estimation of centiles. Further, in our study, the inclusion criteria resulted in a more general reference for birth weight, describing the variation in birth weight within a reference population and did not delineate variation that can be considered ‘ideal’ or of ‘desirable targets’. Despite these limitations, our study will add to the existing scanty literature on birth weight distributions for twin births and will provide basis for future epidemiological studies on twins from this region.

The use of population specific birth weight centile curves will better aid both the clinician and researcher in the assessment of the birth weight of twins. Further, we recommend that assessment in twins consider placental chorionicity. The charts will provide a benchmark to examine the birth weight of twins in relation to other twins born of same gestational age, and would serve as a baseline for future epidemiological research studies. Future work will be to assess whether the infants identified in this way are those with high risk for poor perinatal outcomes, such as stillbirth and neonatal death.