Normal growth and development of twins are important for the long-term health of the children. The purpose of this study 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. This knowledge may lead to interventions that could minimize or prevent pregnancy and newborn health problems in the future.

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. Ultrasound measurements at each follow-up visit included standard fetal biometrics ((BPD, HC, AC, FL); estimated fetal weight was calculated using a Hadlock formula. 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 in the Fetal Growth Studies-Singletons cohort.

In addition to 2-D ultrasound measures for analyzing fetal growth, this study recorded maternal anthropometrics and a brief interview to capture changes in health status, stress and depression, and physical activity at each follow-up visit. Four blood draws at baseline and at targeted gestational ages for the eventual identification of biomarkers indicative of fetal growth were collected. After delivery, neonatal anthropometric measures were assessed for each twin, placental biospecimens were collected, and select information about the obstetrical course and newborn status were abstracted from medical charts. These exposures and outcomes provide opportunities to further 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.

Biospecimen Description:

• Maternal blood sample (serum, plasma, white blood cells, red blood cells and PAXgene RNA) at enrollment, 1st, 3rd, 5th follow-up visits, and delivery
• Cord blood and placentas for consenting women collected at delivery
• Placenta sample or buccal swab (determining zygosity by DNA probe for same sex pairs), collected at delivery

INCLUSION CRITERIA:

• Twin, viable pregnancy
• 8+0 13+6 weeks of gestation
• Maternal age 18 - 45 years
• Spontaneous pregnancy or pregnancy from ovulation induction or in-vitro fertilization with known date of transfer
• Pregnancy from egg donor or embryo donor (record if anonymous or known source)
• LMP-date and crown-rump length measurement match within certain number of days according to a standard scheme (for larger twin)
• Expect to deliver at one of the participating hospitals

EXCLUSION CRITERIA:

• Fetal reduction (medically induced only)
• Congenital anomalies (structural or chromosomal)
• Either twin with an increased nuchal translucency (>99th percentile for crown-rump length; i.e. 3.5 mm or more)
• Monochorionic twins
• Crown-rump length discordancy >10% in dichorionic twin pregnancies

Women with dichorionic twin gestations, irrespective of the mode of conception, maternal medical history or obesity status, were enrolled from eight U.S. clinical centers. Women were enrolled between 8w0d and 13w6d, and accurate dating was required for enrollment. A total of 171 women were recruited, with 152 gestations resulting in live birth.