We investigated body mass index (BMI) and weight gain among pregnant women (ages 14 to 25) and assessed the relationship of BMI and weight gain on birth outcomes. We performed a secondary analysis of 841 women enrolled in a randomized controlled trial receiving prenatal care in two university-affiliated clinics. Almost half the patients were overweight or obese. An average of 32.3 +/- 23.6 pounds was gained in pregnancy with only 25.3% gaining the recommended weight and over half overgaining. Weight gain had a significant relationship to birth weight. Multivariate analysis showed that prepregnancy BMI but not weight gain was a significant predictor of cesarean delivery (odds ratio [OR] 1.91, confidence interval [CI] 1.24 to 2.69, P < 0.0001). When large-for-gestational-age infants were removed from the analysis, there was still a significant effect of BMI on cesarean delivery (OR 1.76, CI 1.17 to 2.66, P = 0.007) but not of weight gain (OR 1.45, CI 0.94 to 2.17, P = 0.093). Prepregnancy BMI is a more significant predictor of cesarean delivery than pregnancy weight gain in young women.