OBJECTIVE: To establish optimal gestational weight gain for each maternal body mass index (BMI) category based on significant risk estimates of adverse maternal and fetal outcome. METHODS: The study population consisted of 298,648 singleton pregnancies delivered in Sweden between January 1, 1994, and December 31, 2004. The number of individuals in each weight gain class was compared with the number of individuals in all other weight gain classes in the same BMI group with regard to adverse maternal and fetal outcome. Odds ratios were calculated after suitable adjustments. RESULTS: The optimal gestational weight gain in women by prepregnancy BMI was 9-22 lb (4-10 kg) for BMI less than 20, 5-22 lb (2-10 kg) for BMI 20-24.9, less than 20 lb (less than 9 kg) for BMI 25-29.9, and less than 13 lb (less than 6 kg) for BMI of 30 or more. CONCLUSION: The gestational weight gain limits for BMI categories determined in this large population-based cohort study from Swedish Medical Registers showed that a decreased risk of adverse obstetric and neonatal outcomes was associated with lower gestational weight gain limits than was earlier recommended, especially among obese women. © 2007 The American College of Obstetricians and Gynecologists.