Combined Influence of Gestational Weight Gain and Estimated Fetal Weight on Risk Assessment for Small- or Large-for-Gestational-Age Birth Weight: A Prospective Cohort Study

Objectives: To evaluate the frequency with which gestational weight gain and estimated fetal weight do not track across gestation and to assess the risk of small-for-gestational-age (SGA) and large-for-gestational-age (LGA) birth weight as a function of tracking. Methods: This study included a pregnancy cohort (2009–2013) of 2438 women from 4 racial/ethnic groups in the United States. We calculated race- and trimester-specific gestational weight gain and estimated fetal weight z scores. The prevalence of how often gestational weight gain and estimated fetal weight did not or did directly track was examined by grouping z scores into measure-specific categories (<−1 SD, −1 to + 1 SD, and >1 SD) and then examining 2-measure combinations. Trimester-specific relative risks for SGA and LGA births were estimated with a gestational weight gain and estimated fetal weight z score interaction. We estimated coefficients for selected gestational weight gain and estimated fetal weight values (–1 SD, 0 SD, and +1 SD) compared with the referent of 0 SD for both measures. Small and large for gestational age were calculated as birth weight below the 10th and at or above the 90th percentiles, respectively. Results: Gestational weight gain and estimated fetal weight were within 1 SD 55.5%, 51.5%, and 48.2% of the time in the first, second, and third trimesters, respectively. There was no significant interaction between gestational weight gain and estimated fetal weight on the risk of SGA in the first and second trimesters (interaction term P =.48; P =.79). In the third trimester, there was a significant interaction (P =.002), resulting in a 71% (95% confidence interval, 1.45–2.02) increased risk of SGA when estimated fetal weight was low and gestational weight gain was high. These relationships were similar for the risk of LGA. Conclusions: Deviations in either measure, even in the presence of average gestational weight gain or estimated fetal weight, still suggest an increased risk of SGA and LGA.