Objective: To examine the relationship between both hospital and provider case volume and obstetric complication rates in U.S. hospitals. Methods: This was a nationwide retrospective cohort study of women admitted to acute care U.S. hospitals for childbirth in 2007. We examined four categories of maternal complications (lacerations, hemorrhage, infections, and thromboses) and created a composite measure. We examined the relationship of hospital volume, provider volume, and odds of complications. Results: We found no consistent relationship between hospital volume and rates of maternal complications. In contrast, we found that women cared for by providers in the lowest quartile of provider volume (fewer than seven deliveries per year) had a 50% higher odds of complications compared with women cared for by obstetricians in the highest quartile (odds ratio 1.5, 95% confidence interval, 1.3-1.7, P<.001). Each of the individual complications occurred more frequently among the lowest-volume providers compared with others. Models that adjusted for hospital characteristics and cesarean delivery rate had only modest effects. Conclusion: Individual providers with a low volume of deliveries have greater maternal complication rates than providers with a high volume. If volume is causally related to lower complication rates, strategies for improving care for women during childbirth may include selective referral to higher-volume providers or additional training for low-volume providers to ensure better outcomes.