The overarching goals of the NICHD Fetal Growth Studies are to 1) determine optimal growth standards for 4 racial/ethnic groups of fetuses and 2) to identify factors associated with decrements in expected fetal growth. Related goals are in developing algorithms for better estimating fetal weight and in determining the optimal timing of delivery. A major accomplishment was in the development of a database for the storage of 2D and 3D ultrasonology images and accompanying biometric data. This database is a valuable resource for follow on work aimed at determining specific growth dimensions such as bone length, volumes or organ size. Two overarching findings have been reported in separate publications. First, we found that estimated fetal weight differed significantly by self reported maternal race/ethnicity after 20 weeks gestation. Specifically at 39 weeks, the 5th, 50th, and 95th percentiles were 2790, 3505, and 4402 grams for White, 2633, 3336, and 4226 grams for Hispanic, 2621, 3270, and 4078 grams for Asian, and 2622, 3260, and 4053 grams for Black women (adjusted global p<0.001). As such, we found that using a fetal growth standard based solely on the White group erroneously classifies as much as 15% of non-White fetuses as growth-restricted (estimated fetal weight < 5th percentile). Thus, racial/ethnic-specific standards may improve the precision in evaluating fetal growth. A second key finding stemmed from the comparison of women with singleton and twin pregnancies relative to fetal growth. We found asymmetric growth patters for twin relative to singletons arising in the second trimester that support the premise that the intrauterine environment becomes constrained in its ability to sustain growth in twin fetuses.
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