Dichorionic twin trajectories: the NICHD Fetal Growth Studies.

Area of Science:

• Maternal-fetal medicine
• Prenatal diagnostics
• Fetal growth research

Background:

• Accurate twin growth assessment requires controlled ultrasound data, which is scarce for contemporary populations.
• There is ongoing debate regarding the use of singleton growth benchmarks for twin fetal growth evaluation.

Purpose of the Study:

• To empirically define fetal growth trajectories for dichorionic twins using longitudinal ultrasonography.
• To compare these twin growth trajectories against a singleton growth standard.

Main Methods:

• Prospective cohort study of 171 women with twin gestations across 8 US sites (2012-2013).
• Longitudinal 2D ultrasonography with random assignment to serial scanning schedules.
• Growth curves estimated using linear mixed models with cubic splines; statistical comparisons adjusted for covariates.

Main Results:

• Twin fetal growth trajectories, particularly abdominal circumference and estimated fetal weight, diverged significantly from singletons starting at 32 weeks gestation.
• Biparietal diameter was smaller in twins from 34-36 weeks; head circumference/abdominal circumference ratio increased in twins from 33 weeks, indicating asymmetric growth.
• At 35 weeks, 34-38% of twins would be classified as small for gestational age if using a singleton standard.

Conclusions:

• The observed asymmetric growth pattern in twins, evident from 32 weeks, suggests intrauterine environmental constraints.
• Current singleton growth standards may lead to over-classification of twins as small for gestational age, necessitating specific twin growth benchmarks.