Objective. To analyze risk factors for peroperative excessive blood loss at cesarean delivery. Design. Case-control study. Setting. Twenty-four of 26 maternity units in Norway with at least 500 expected deliveries per year. Sample. A total of 2,778 women having singleton deliveries and participating in the Norwegian Breakthrough Project on Cesarean Section. Methods. Elective and emergency operations were analyzed separately with extensive blood loss defined as hemorrhage > 1000 ml with controls defined as bleeding < 500 ml. All analyzes were done in SPSS (version 16.0) with chi-squared tests and logistic regression. Main outcome measures. Adjusted odds ratios (aOR) of extensive peroperative bleeding. Results. The prevalence of excessive blood loss differed between women undergoing elective (2.1%) and emergency cesarean deliveries (3.3%). Among maternal factors, chronic maternal diseases, pregnancy and delivery related conditions, placenta previa (aOR 19.7; 95% CI 5.4–72.2) and transverse lie (aOR 4.9; 95% CI 0.9–26.5) were the only risk factors for extensive blood loss in elective operations, whereas placenta previa (aOR 8.4; 95% CI 2.4–29.9), placental abruption (aOR 2.0; 95% CI 2.0–14.5), intervention at full cervical dilation (aOR 3.2; 95% CI 1.4–7.1) and high BMI (aOR 3.4; 95% CI 1.6–7.2) were risks in emergency operations. Conclusion. The different risk pattern for excessive bleeding in cesarean deliveries should be recognized when planning available obstetric competence for surgery.