OBJECTIVE: Our purpose was to study the impact of labor onset and delivery mode on the risk of severe postpartum hemorrhage. STUDY DESIGN: This was a population-based study of 307,415 mothers who were registered in the Medical Birth Registry of Norway from 1999-2004. RESULTS: Severe postpartum hemorrhage occurred in 1.1% of all mothers and in 2.1% of those mothers with previous cesarean section delivery (CS). Compared with spontaneous labor, hemorrhage risk was higher for induction (odds ratio [OR], 1.71; 95% confidence interval [CI], 1.56-1.88) and prelabor CS (OR, 2.05; 95% CI, 1.84-2.29). The risk was 55% higher for emergency CS and half that for vaginal deliveries (OR, 0.48; 95% CI, 0.43-0.53), compared with prelabor CS. The highest risk was for emergency CS after induction in mothers with previous CS (OR, 6.57; 95% CI, 4.25-10.13), compared with spontaneous vaginal delivery in mothers with no previous CS. CONCLUSION: Induction and prelabor CS should be practiced with caution because of the increased risk of severe postpartum hemorrhage.