© 2017 American Medical Association. All rights reserved. IMPORTANCE Despite the increasing prevalence of pregravid obesity, systematic evaluation of the association of maternal obesity with fetal growth trajectories is lacking. OBJECTIVE To characterize differences in fetal growth trajectories between obese and nonobese pregnant women, and to identify the timing of any observed differences. DESIGN, SETTING, AND PARTICIPANTS The Eunice Kennedy Shriver National Institute of Child Health and Human Development Fetal Growth Studies-Singletons study enrolled cohorts of pregnant women at 12 US health care institutions. Obese women (with prepregnancy body mass index > 30) and nonobese women (prepregnancy body mass indexes, 19-29.9) without major chronic diseases were recruited between 8 weeks and 0 days' gestation and 13 weeks and 6 days' gestation. A mixed longitudinal randomization scheme randomized participants into 1 of 4 schedules for 2-dimensional and 3-dimensional ultrasonograms to capture weekly fetal growth data throughout the remainder of their pregnancies. MAIN OUTCOMES AND MEASURES On each ultrasonogram, fetal humerus length, femur length, biparietal diameter, head circumference, and abdominal circumference were measured. Fetal growth curves were estimated using linear mixed models with cubic splines. Median differences in the fetal measures at each gestational week of the obese and nonobese participants were examined using the likelihood ratio andWald tests after adjustment for maternal characteristics. RESULTS The study enrolled 468 obese and 2334 nonobesewomen between 8weeks and 0 days' gestation and 13weeks and 6 days' gestation. After a priori exclusion criteria, 443 obese and 2320 nonobesewomen composed the final cohort. Commencing at 21weeks' gestation, femur length and humerus lengthwere significantly longer for fetuses of obesewoman than those of nonobesewomen. Differences persisted in obese and nonobese groups through 38 weeks' gestation (median femur length, 71.0 vs 70.2 mm; P = .01; median humerus length, 62.2 vs 61.6 mm; P = .03). Averaged across gestation, head circumferencewas significantly larger in fetuses of obesewomen than those of nonobesewomen (P = .02). Fetal abdominal circumferencewas not greater in the obese cohort than in the nonobese cohort butwas significantly larger than in fetuses of normal-weightwomen (with body mass indexes between 19.0-24.9) commencing at 32weeks (median, 282.1 vs 280.2 mm; P = .04). Starting from 30 weeks' gestation, estimated fetalweightwas significantly larger for the fetuses of obesewomen (median, 1512 g [95%CI, 1494-1530 g] vs 1492 g [95%CI, 1484-1499 g]) and the difference grew as gestational age increased. Birthweightwas higher by almost 100 g in neonates born to obesewomen than to nonobesewomen (mean, 3373.2 vs 3279.5 g). CONCLUSIONS AND RELEVANCE As early as 32 weeks' gestation, fetuses of obese women had higher weights than fetuses of nonobese women. The mechanisms and long-term health implications of these findings are not yet established.