Dichorionic twin trajectories: the NICHD Fetal Growth Studies

Abstract

Background Systematic evaluation and estimation of growth trajectories in twins require ultrasound measurements across gestation, 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 using the same benchmarks as singleton growth. Objectives Our objective was to empirically define the trajectory of fetal growth in dichorionic twins using longitudinal two-dimensional ultrasonography and to compare the fetal growth trajectories for dichorionic twins with those based on a growth standard developed by our group for singletons. Study Design A prospective cohort of 171 women with twin gestations was recruited from eight U.S. sites from 2012 to 2013. After an initial sonogram at 11w0d–13w6d where dichorionicity was confirmed, women were randomized to one of two serial ultrasonology schedules. Growth curves and percentiles were estimated using linear mixed models with cubic splines. Percentiles were compared statistically at each gestational week between the twins and 1,731 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 using 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 where there were significant global tests for differences between twins and singletons, we tested for week-specific differences using Wald tests computed at each gestational age. In a separate analysis, we evaluated the degree of reclassification in small for gestational age, defined as below the 10th percentile that would be introduced if fetal growth estimation for twins was based upon an unweighted singleton standard. Results Women underwent a median of 5 ultrasounds. The 50th percentile abdominal circumference and estimated fetal weight trajectories of twin fetuses diverged significantly beginning at 32 weeks, while biparietal diameter in twins was smaller from 34 through 36 weeks. 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, indicating a comparatively asymmetric growth pattern. At 35 weeks, 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 and 2180, 2567, and 3022 g for the singletons. At 32 weeks, 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 using a singleton, non-Hispanic white standard. By 35 weeks, 38% of...

Maternal weight gain and associations with longitudinal fetal growth in dichorionic twin pregnancies: a prospective cohort study

“…The study procedures have been previously described (11). Institutional review board approval was obtained by all participating institutions, the data coordinating center, and the NICHD.…”

“…Women were enrolled between 8 wk 0 d and 13 wk 6 d. After an initial sonogram between 11 wk 0 d and 13 wk 6 d, women were randomly assigned to 1 of 2 follow-up schedules (11). Each follow-up schedule consisted of #6 sonograms scheduled at prespecified gestational weeks.…”

“…Each follow-up schedule consisted of #6 sonograms scheduled at prespecified gestational weeks. The only difference between the 2 follow-up schedules was the targeted gestational age to ensure that the research visits were equally distributed across the gestation to address the main aims of assessing longitudinal fetal growth in twins in the NICHD Fetal Growth Studies -Twins study (11).…”

“…Fetal measurements were obtained according to a standardized ultrasound protocol (11,14). Fetal measurements relevant to the current analysis included biparietal diameter (BPD, outer-toinner), head circumference (HC), AC, humerus length (HL), and femur length (FL), all measured in millimeters.…”

“…With the use of data from a prospective cohort of women with dichorionic twin gestations (11) followed from the first trimester onward, we sought to address these critical data gaps related to maternal weight gain and fetal growth in twin pregnancies. With the use of repeated maternal weight gain and ultrasound data, our aims were 1) to evaluate the association between gestational weight gain and twin fetal growth across different time periods of gestation, and 2) to assess if such associations were specific to certain fetal biometry variables.…”