Objective: To estimate the risk of cesarean delivery due to excess prepregnancy body mass index (BMI) in a multistate, US population-based sample. Methods: We analyzed data from the population-based Pregnancy Risk Assessment Monitoring System (PRAMS) on 24,423 nulliparous women with single, term infants delivered between 1998 and 2000 in 19 states. We calculated BMI from self-reported weight and height. We assessed interactions between prepregnancy BMI and other risk factors. We estimated weighted relative risks and 95% confidence intervals for the association between prepregnancy BMI and cesarean section from multiple logistic regression models adjusting for demographic and medical risk factors from the PRAMS questionnaire or birth certificates. Results: The incidence of cesarean delivery increased with increased prepregnancy BMI, from 14.3% (0.8 standard error (SE)) for lean women (BMI < 19.8) to 42.6% (2.0 SE) for very obese women (BMI ≥ 35). The risk of cesarean section differed by presence of any medical, labor and/or delivery complication. Among women with any complication, the estimated adjusted RR for cesarean delivery was 1.1 (95% confidence interval (CI) 1.0–1.2) among overweight women, 1.3 (95% CI 1.1–1.4) among obese women, and 1.4 (95% CI 1.2–1.6) among very obese women compared with normal weight women. Among women without any complications, the estimated adjusted RR was 1.4 (95% CI 1.0–1.8) among overweight women, 1.5 (95% CI 1.1–2.1) among obese women, and 3.1 (95% CI 2.3–4.8) among very obese women. Conclusion: Excess prepregnancy weight increases the risk of cesarean delivery among nulliparous women giving birth to single, term infants, especially among very obese women without any complications.