OBJECTIVE: To evaluate pregnancy outcomes according to 2009 Institute of Medicine (IOM) gestational weight gain guidelines. METHODS: This study is a secondary analysis of a pre-eclampsia prevention trial among nulliparas carrying singletons. Odds ratios and 95% confidence intervals (adjusted for maternal age, race, smoking, and treatment group) were calculated based on total weight gain below or above the IOM guidelines stratified by prepregnancy body mass index (BMI). The referent group was weight gain within the guidelines. RESULTS: Of 8,293 pregnancies, 9.5% had weight gain below, 17.5% within, and 73% above IOM guidelines. With excess weight gain, all BMI categories had an increased risk of hypertensive disorders; normal weight and overweight women also had increased risk of cesarean delivery and neonatal birth weight at or above the 90th centile but a decreased risk of weight below the 10th centile. There were no consistent associations with insufficient weight gain and adverse outcomes. CONCLUSION: Excess weight gain was prevalent and associated with an increased risk of hypertensive disorders, cesarean delivery, and large-for-gestational-age neonates. © 2013 by The American College of Obstetricians and Gynecologists.