The NICHD Fetal Growth Study: Twin Gestations

Fetal growth trajectories in dichorionic twin pregnancies were created using two- and selected three-dimensional ultrasound measures. Measures included maternal anthropometric measurements and 2D fetal biometry (BPD, HC, AC, FL) using standardized protocols.

Women were recruited during the first trimester and followed until delivery. Each woman had seven scheduled ultrasound examinations, one at enrollment and six during follow-up visits. Women were randomized to receive sonograms according to one of the following two schedules: schedule A: 16, 20, 24, 28, 32 and 35 weeks or schedule B: 18, 22, 26, 30, 34 and 36 weeks.

The primary analysis compared fetal growth trajectories for dichorionic twins with 1,737 singleton gestations included in the NICHD Fetal Growth Studies - Singleton standard. For modeling twin trajectories, we used linear mixed models with a cubic spline mean structure and a random effects structure that included linear, quadratic, and cubic random effects for the twin pair and an intercept term for the individual fetus within twin pair. The linear mixed models were also used to test for overall differences (i.e., global tests) between the twin and singleton trajectories (for EFW and other measurements) using likelihood ratio tests of interaction terms between spline mean structure terms and twin-singleton indicator variables.

Using small-for-gestational age as a proxy for growth restriction, we evaluated the percentage of dichorionic twins who would be classified below the 10th percentile using the study-generated singleton non-Hispanic white standard.

Longitudinal data will compare singleton and twin gestations with respect to maternal biomarkers of fetal growth, well-being and maternal nutritional status using banked biospecimens to determine if the biomarkers ranges established for singletons are applicable to twin gestations.

Pending the results of a validation study, measures of fetal fat and lean body mass from 2D ultrasound and/or fractional volumes from 3D will be assessed.

Twin gestations offer an opportunity to differentiate genetic from environmental etiologies of developmental disorders, and to gain insight by investigating the role of shared factors versus factors affecting each of the twins independently. This involves comparing results of within cohort versus within twin pairs. The shared factors include maternal nutrition and gestational age, and the non-shared factors are those that affect each twin (e.g., placental placement, growth trajectory).

The available bank of de-identified specimens will be used to add to our understanding of the determinants associated with fetal growth trajectories or adverse outcomes. The repository will be important for exploring possible epigenetic mechanisms and other specific hypotheses investigators may have.

Placental samples for zygosity determination were obtained for all like-sex twin pairs. In the event that placental samples were not able to be obtained for like-sex twin pairs, a buccal swab from each infant was obtained prior to discharge, preferably in the first 48 hours of life. Both placenta zygosity samples and buccal swab samples were sent in real time to the Columbia Placenta Pathology Laboratory for zygosity testing. The research nurse provided women her with zygosity results when the information was available within three months following delivery.

Information on mode of conception was collected during the enrollment interview.

The gross anatomy of the placentas (weight, cord length, diameter of the cord, and other gross morphology) were documented by the clinical site's participating pathologist. Photographs of placentas were taken by trained research personnel. Placental samples were obtained according to a standardized protocol. Eventual analysis may include proteomics, DNA analyses (e.g., comparative genomic hybridization, genomics, and assessment of the methylome), RNA expression, and microRNA levels. Also, analysis may include immunocytochemistry, histopathologic and biochemical / molecular studies. Once the research specimens were obtained, the remaining placenta was submitted to the clinical pathology department for processing and analysis.