Dichorionic twin trajectories: the NICHD Fetal Growth Studies

Abstract

Background: Systematic evaluation and estimation of growth trajectories in twins require ultrasound measurements across gestation that are performed in controlled clinical settings. Currently, there are few such data for contemporary populations. There is also controversy about whether twin fetal growth should be evaluated with the use of the same benchmarks as singleton growth.

Objectives: Our objective was to define the trajectory of fetal growth in dichorionic twins empirically using longitudinal 2-dimensional ultrasonography and to compare the fetal growth trajectories for dichorionic twins with those based on a growth standard that was developed by our group for singletons.

Study design: A prospective cohort of 171 women with twin gestations was recruited from 8 US sites from 2012-2013. After an initial sonogram at 11 weeks 0 days-13 weeks 6 days of gestation during which dichorionicity was confirmed, women were assigned randomly to 1 of 2 serial ultrasonography schedules. Growth curves and percentiles were estimated with the use of linear mixed models with cubic splines. Percentiles were compared statistically at each gestational week between the twins and 1731 singletons, after adjustment for maternal age, race/ethnicity, height, weight, parity, employment, marital status, insurance, income, education, and infant sex. Linear mixed models were used to test for overall differences between the twin and singleton trajectories with the use of likelihood ratio tests of interaction terms between spline mean structure terms and twin-singleton indicator variables. Singleton standards were weighted to correspond to the distribution of maternal race in twins. For those ultrasound measurements in which there were significant global tests for differences between twins and singletons, we tested for week-specific differences using Wald tests that were computed at each gestational age. In a separate analysis, we evaluated the degree of reclassification in small for gestational age, which was defined as <10th percentile that would be introduced if fetal growth estimation for twins was based on an unweighted singleton standard.

Results: Women underwent a median of 5 ultrasound scans. The 50th percentile abdominal circumference and estimated fetal weight trajectories of twin fetuses diverged significantly beginning at 32 weeks of gestation; biparietal diameter in twins was smaller from 34-36 weeks of gestation. There were no differences in head circumference or femur length. The mean head circumference/abdominal circumference ratio was progressively larger for twins compared with singletons beginning at 33 weeks of gestation, which indicated a comparatively asymmetric growth pattern. At 35 weeks of gestation, the average gestational age at delivery for twins, the estimated fetal weights for the 10th, 50th, and 90th percentiles were 1960, 2376, and 2879 g for dichorionic twins, respectively, and 2180, 2567, and 3022 g for the singletons, respectively. At 32 weeks of gestation, the initial week when the mean estimated fetal weight for twins was smaller than that of singletons, 34% of twins would be classified as small for gestational age with the use of a singleton, non-Hispanic white standard. By 35 weeks of gestation, 38% of twins would be classified as small for gestational age.

Conclusion: The comparatively asymmetric growth pattern in twin gestations, initially evident at 32 weeks of gestation, is consistent with the concept that the intrauterine environment becomes constrained in its ability to sustain growth in twin fetuses. Near term, nearly 40% of twins would be classified as small for gestational age based on a singleton growth standard.

Keywords: dichorionic; estimated fetal weight; fetal growth; twin.

Conflict of interest statement

Disclosure: The authors report no conflict of interest.

Published by Elsevier Inc.

Figures

Figure 1. Flow diagram for study participants in the NICHD Fetal Growth Studies - Twin Gestations

All data from women were included in the analysis up until the time that they had an event (e.g. delivery, deactivation, etc.). aGestational age (range) in weeks in which the visits occurred. bThe numbers in the parentheses indicate women who missed the visit. cReasons for deactivation: Voluntary termination of both fetuses (n=1), Voluntary termination of one fetus (n=1), miscarriage of one fetus (n=1), moved (n=1), refusal to continue (n=1). dReasons for deactivation: Voluntary termination of one fetus (n=2), miscarriage/stillbirth (intrapartum fetal death) of both twins (n=2), antepartum fetal death of both twins (n=1) eReasons for deactivation: Moved (n=1) fReasons for deactivation: Antepartum fetal death (n=1), refusal to continue (n=1)

Figure 2. Distribution of estimated fetal weight by number of fetuses and gestation, NICHD Fetal Growth Studies – Twin Gestations

Estimated 10th, 50th and 90th percentiles for fetal weight for dichorionic twin gestations and singleton gestations included in the standard, as estimated from linear mixed models with log-transformed outcomes and cubic splines. For an overall comparison between twins and singletons fetal trajectories, the singleton standards were weighted to have the same racial/ethnic distribution as observed in the twin cohort. EFW, estimated fetal weight; GA, gestational age

Figure 3. Distribution of fetal anthropometric measurements by number of fetuses and gestation, NICHD Fetal Growth Studies – Twin Gestations

Estimated 10th, 50th and 90th percentiles for the fetal anthropometric parameters for dichorionic twin gestations and singleton gestations included in the standard (a–e), as estimated from linear mixed models with log-transformed outcomes and cubic splines. GA, gestational age

Figure 4. Percentage of dichorionic twin fetuses below the 10th percentile of the Non-Hispanic White singleton standard

Percentage of twin fetuses below the 10th percentile of the Non-Hispanic White singleton standard by gestational age. The difference between the twin-specific curves and the 0.10 line reflect the amount of classification attributed to using the Non-Hispanic White singleton standard.