PROJECT SUMMARY Maternal obesity and excess weight gain in pregnancy are the leading causes of maternal and fetal morbidity. Randomized trials in women with obesity have shown that lifestyle interventions in pregnancy produce modest reductions in gestational weight gain, yet unfortunately fail to produce clinically significant improvements in maternal and child outcomes as long hoped. Several well-defined prospective cohort studies suggest that for women with obesity, weight maintenance during pregnancy – and specifically loss in fat mass – may be required to reduce adverse maternal and child outcomes. However, there are no randomized controlled trials testing the health effects of weight maintenance and significant maternal fat mass loss during pregnancy in women with obesity. Our novel preliminary energy balance and body composition data showed that weight maintenance during pregnancy could be achieved with careful timing of caloric restriction during the second trimester and eucaloric goals during the 3rd trimester of pregnancy. Based on these and our other preliminary data, we propose a highly rigorous proof-of-principle trial to determine the effects of a 6-month gestational intervention with calorie restriction and food provision to promote maternal weight maintenance and fat loss in women with grades 2 and 3 obesity. Pregnant women with grades 2 and 3 obesity (N = 100; 30% Hispanic; 30% Black) will be randomized to usual care or a gestational fat mass loss (FML) intervention. The principal aims are to determine the effects of FML during pregnancy on changes in 1) maternal weight, fat mass, and cardiometabolic risk factors; and, 2) safety measures, including fetal and neonatal growth. All women will be assessed at baseline (13-16 weeks), 27-29 weeks, and 35-37 weeks gestation and at 2 weeks postpartum; safety measures are collected every 4 weeks throughout the trial. This project is significant because findings have the potential to set the stage for a revision in national recommendations that currently encourage weight gain during pregnancy in women with severe obesity; findings may lead to new guidelines that encourage practitioners to not delay intensive weight management treatment in women with obesity who become pregnant and present for healthcare. The study is also innovative; there have been no randomized controlled trials evaluating the effects of a an intervention intensive enough to actually achieve weight maintenance during pregnancy and provide much needed safety and efficacy data to inform practice recommendations for diverse women with severe obesity who lack effective treatment options.