Comparison of fetal growth estimates based on birth weight and ultrasound references

Abstract

Objective: To compare three different methods for modeling fetal weight gain during the third trimester of pregnancy.Methods: Ultrasound and live birth weight data were used to constructthree models for defining fetal growth during the third trimester: longitudinal ultrasound estimates of fetal weight obtained serially at 3-4 week intervals in 50 uncomplicated, well-dated pregnancies between19 and 40 weeks' gestation; cross-sectional ultrasound estimates of fetal weight obtained from 2018 ultrasound examinations of singleton, non-anomalous fetuses between 24 and 39 weeks' gestation; and cross-sectionalbirth weight data obtained from 9553 live singleton, non-anomalous neonates between 24 and 43 completed weeks. Analysis was performed by pairwise partial f test to compare regression curves and ztest for comparison of mean weekly weight gain. A value of p < 0.05 was accepted for significance.Results: Derived regression lines depicting fetal size across gestation were significantlydifferent from each other (f tests, p < 0.05). Estimates of mean fetal weight were significantly different between the three different models at specific gestational ages. Significant weeklyvariations in fetal weight gain were observed within the raw cross-sectional data sets, both for ultrasound-estimated fetal weight (range 91-278 g/week) and birth weight (65-309 g/week).Conclusions:Each of the methods used to model normal fetal weight gain in the third trimester defined a distinct pattern of fetal growth. Normal fetal growth, defined longitudinally, was most closely matched by a combinationof cross-sectional ultrasound-derived estimated fetal weight in preterm gestation below 34 weeks' gestation and live birth weight at or beyond 34 weeks.