RESULTS: There was an increased prevalence of postpartum hemorrhage over the study period associated primarily with changes in maternal characteristics. The risk of atonic uterine hemorrhage increased rapidly with increasing BMI. There was a twofold increased risk in obesity class III (1.8%). No association was found between postpartum hemorrhage with retained placenta and maternal obesity. There was an increased risk for postpartum hemorrhage for women with a BMI of 40 or higher (5.2%) after normal delivery (odds ratio [OR] 1.23, 95% confidence interval [CI] 1.04 –1.45]) compared with normal-weight women (4.4%) and even more pronounced (13.6%) after instrumental delivery (OR 1.69, 95% CI 1.22–2.34) compared with normal-weight women (8.8%). Maternal obesity was a risk factor for the use of heparin-like drugs (OR 2.86, 95% CI 2.22–3.68)