A conditional standard for the customized fetal growth trajectory in twin pregnancy

Abstract

BackgroundThe difference between the birthweights of twins and singletons grows with advancing gestation. Although many fetal weight standards based on ultrasound have been created for tracking fetal growth in twin pregnancies, their applicability to other groups is limited by the fact that they are population-specific. ObjectivesTo generate conditional centiles of growth assessment in twin fetuses and compare them with other population-based growth standards for singleton and twin fetuses. Study DesignA retrospective longitudinal study of the ultrasound-based estimated fetal weight (EFW) dataset of normal twin fetuses delivered after 34 weeks of gestation was conducted. Linear mixed effects models, which adjusted for maternal characteristics, fetal gender, and chorionicity, were used to construct the relationship between EFW and gestational age. The EFW reference values were calculated using conditional centile based on the EFW at an earlier gestational age. To compare our results with previous studies, fetal growth curves were generated using a formula we created which included maternal characteristics and the EFW at 24 weeks of gestation in these studies. In a subgroup analysis of our low-risk twin babies born at full term, we calculated the number of fetuses that were reclassified as being in the bottom 10th percentile using each of the previous population-based standard. Results2644 ultrasounds with a median of 4 scans per fetus from 572 twin pregnancies were included in this analysis. Thirty six percent of the fetuses were monochorionic. Maternal age, body mass index, and the interaction between fetal gender and chorionicity were significantly associated with EFW. The predicted growth curves matched the growth standard for twins. In our low-risk group, when the singleton standard was used, the incidence of EFW 10th percentile was above 20% from gestational week 24–38, and this incidence varied when reclassified using other population-based twin standards. ConclusionsThis conditional growth chart was specifically designed to assess fetal growth in twins, but it is generalizable to other populations.