Objective Because the diagnosis of postpartum hemorrhage (PPH) depends on the accoucheur's subjective estimate of blood loss and varies according to mode of delivery, we examined temporal trends in severe PPH, defined as PPH plus receipt of a blood transfusion, hysterectomy, and/or surgical repair of the uterus. Study Design We analyzed 8.5 million hospital deliveries in the US Nationwide Inpatient Sample from 1999 to 2008 for temporal trends in, and risk factors for, severe PPH, based on International Classification of Diseases, 9th revision, clinical modification diagnosis and procedure codes. Sequential logistic regression models that account for the stratified random sampling design were used to assess the extent to which changes in risk factors explain the trend in severe PPH. Results Of the total 8,571,209 deliveries, 25,906 (3.0 per 1000) were complicated by severe PPH. The rate rose from 1.9 to 4.2 per 1000 from 1999 to 2008 ( P for yearly trend < .0001), with increases in severe atonic and nonatonic PPH, due especially to PPH with transfusion, but also PPH with hysterectomy. Significant risk factors included maternal age ≥35 years (adjusted odds ratio aOR, 1.5; 95% confidence interval CI, 1.5–1.6), multiple pregnancy (aOR, 2.8; 95% CI, 2.6–3.0), fibroids (aOR, 2.0; 95% CI, 1.8–2.2), preeclampsia (aOR, 3.1; 95% CI, 2.9–3.3), amnionitis (aOR, 2.9; 95% CI, 2.5–3.4), placenta previa or abruption (aOR, 7.0; 95% CI, 6.6–7.3), cervical laceration (aOR, 94.0; 95% CI, 87.3–101.2), uterine rupture (aOR, 11.6; 95% CI, 9.7–13.8), instrumental vaginal delivery (aOR, 1.5; 95% CI, 1.4–1.6), and cesarean delivery (aOR, 1.4; 95% CI, 1.3–1.5). Changes in risk factors, however, accounted for only 5.6% of the increase in severe PPH. Conclusion A doubling in incidence of severe PPH over 10 years was not explained by contemporaneous changes in studied risk factors.