The purpose of this study was to define independent risk factors for uterine atony after primary cesarean delivery, and to assess their overall association with atony in the study cohort. Study design: This was a 13- university center prospective observational study. All women who underwent primary cesarean from January 1, 1999 to December 31, 2000 were eligible. Trained and certified research nurses performed systematic data Abstraction. The definition of atony required both the clinical diagnosis and the use of methergine or a prostaglandin preparation. Risk factors for uterine atony were assessed in univariable and multivariable logistic regression analyses, and these analyses then used to inform an assessment of the association of the various risk factors with the occurrence of uterine atony in the overall cohort. Results: Twenty-three thousand, three hundred and ninety pregnancies were analyzed. Uterine atony occurred in 1416 women (6% ). Several variables were independently associated with atony in a multivariable model, including multiple gestation (odds ratio [OR] 2.40, 95% CI 1.95- 2.93), maternal Hispanic race (2.21, 1.90- 2.57), induced or augmented labor for >18 hours (2.23, 1.92- 2.60), infant birth weight >4500 g (2.05, 1.53- 2.69), and clinically diagnosed chorioamnionitis (1.80, 1.55- 2.09). However, because the various risk factors were not very powerful, approximately half of the cases of atony were associated with the 2/3 of women lacking a given risk factor or combination of risk factors. Conclusion: Although certain risk factors and uterine atony were clearly associated, the associations are of limited practical clinical use. The purpose of this study was to define independent risk factors for uterine atony after primary cesarean delivery, and to assess their overall association with atony in the study cohort. Study design: This was a 13- university center prospective observational study. All women who underwent Primary cesarean from January 1, 1999 to December 31, 2000 were eligible. Trained and certified research nurses performed systematic data Abstraction. The definition of atony required both the clinical diagnosis and the use of methergine or a prostaglandin preparation. Risk factors for uterine atony were assessed in univariable and multivariable logistic regression analyzes, and these analyzes then used to inform an assessment of the association of the various risk factors with the occurrence of uterine atony in the overall cohort. Results: Twenty-three thousand, three hundred and ninety pregnancies were analyzed. Uterine atony occurred in 1416 women (6%). Several variables were independently assoc iated with atony in a multivariable model, including multiple gestation (odds ratio [OR] 2.40, 95% CI 1.95-2.93), maternal Hispanic race (2.21, 1.90-2.57), induced or augmented labor for> 18 hours - 2.60), infant birth weight> 4500 g (2.05, 1.53- 2.69), and clinically diagnosed chorioamnionitis (1.80, 1.55-2.09). However, because the various risk factors were not very powerful, approximately half of the cases of atony were associated with the 2/3 of women lacking a given risk factor or combination of risk factors. Conclusion: Although certain risk factors and uterine atony were clearly associated, the associations are limited limited practical clinical use.