OBJECTIVE: To examine whether pregnancy weight gains outside the Institute of Medicine (IOM) recommendations and rates of maternal weight gain are associated with neonatal complications. METHODS: In a cohort of 45,245 women who delivered singletons at Kaiser Permanente Medical Care Program Northern California in 1996-1998 and who did not have gestational diabetes as of 24-28 weeks of gestation, we conducted a nested case-control study with three case groups: macrosomia (birth weight more than 4,500 g, n=391), neonatal hypoglycemia (plasma glucose less than 40 mg/dL, n=328), and hyperbilirubinemia (serum bilirubin 20 mg/dL or more, n=432) and one control group (n=652). Medical records were reviewed to ascertain the woman’s prepregnancy and predelivery weight. RESULTS: Adjusting for age, race-ethnicity, parity, plasma glucose screening value, and difference in weeks between delivery and time when last weight was measured, women who gained more than recommended by the IOM were three times more likely to have an infant with macrosomia (odds ratio [OR] 3.05, 95% confidence interval [CI] 2.19-4.26), and nearly 1.5 times as likely to have an infant with hypoglycemia (OR 1.38, 95% CI 1.01-1.89), or hyperbilirubinemia (OR 1.43, 95% CI 1.06-1.93) than women whose weight gain was in the recommended range. Women who gained less than the IOM recommendations were less likely than women in the recommended range to have an infant with macrosomia (OR 0.38, 95% CI 0.20-0.70), but equally likely to have an infant with hypoglycemia or hyperbilirubinemia. Similar results were obtained using other means of categorizing weight gain during pregnancy. CONCLUSION: Maternal weight gain above the IOM recommendations was associated with an increased risk of the outcomes studied. LEVEL OF EVIDENCE: II-2.