OBJECTIVE:: To estimate whether an organized, consistent program of dietary and lifestyle counseling prevents excessive weight gain in pregnancy. METHODS:: This randomized controlled trial assigned women to receive either an organized, consistent program of intensive dietary and lifestyle counseling or routine prenatal care. The primary study outcome was the proportion of patients whose gestational weight gain was within the Institute of Medicine (IOM) guidelines. Secondary outcomes included mode of delivery, rate of operative vaginal delivery, neonatal weight, and the incidence of preeclampsia, gestational diabetes mellitus (GDM), vaginal/perineal lacerations, and shoulder dystocia. RESULTS:: A total of 100 women were randomized to the study (lifestyle counseling 57, routine prenatal care 43). Baseline demographic characteristics were similar between the study groups. The lifestyle counseling group gained significantly less weight than did the routine prenatal care group (28.7±12.5 lb compared with 35.6±15.5 lb, P≤.01). The routine prenatal care group had significantly more cesarean deliveries due to gfailure to progress (routine prenatal care 58.3% compared with lifestyle counseling 25.0%, P≤.02). Across groups, patients who were not adherent to the IOM guidelines had significantly heavier neonates (adherent 3,203.2±427.2 g compared with not adherent 3,517.4±572.4 g, P<.01). Nulliparous women gained significantly more weight than did parous women (36.5±14.5 lb compared with 27.7±12.7 lb, P<.01). The most predictive factor of IOM adherence was having a normal prepregnancy body mass index. No statistically significant differences were noted between the groups in adherence to IOM guidelines, rate of cesarean delivery, preeclampsia, GDM, operative vaginal delivery, or vaginal lacerations. CONCLUSION:: An organized, consistent program of dietary and lifestyle counseling did reduce weight gain in pregnancy. © 2009 by The American College of Obstetricians and Gynecologists. Published by Lippincott Williams & Wilkins.