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Predictors of Failed Operative Vaginal Delivery in a Contemporary Obstetric Cohort. OBJECTIVE: To identify factors associated with failed operative vaginal delivery in contemporary parturients and evaluate whether these factors can be used to accurately predict failed operative vaginal delivery. METHODS: This was a case-control study of women undergoing a trial of operative vaginal delivery from a low or outlet station between 2005 and 2014. Women in the case group were defined as women who had an attempted operative vaginal delivery but ultimately required cesarean delivery. Women in the control group were defined as women who delivered vaginally. Bivariable and multivariable analyses were performed to determine factors that were independently associated with failed operative vaginal delivery. A receiver operating characteristic curve was created and area under the curve calculated to estimate the predictive capacity of these associations. RESULTS: Of 4,352 women who met inclusion criteria, 2,527 underwent an attempt at operative vaginal delivery using forceps and 1,825 using vacuum. Failure occurred in 272 (6.3%). In bivariable analysis, nulliparity, white race, induction of labor, chorioamnionitis, second stage 2 hours or greater, fetal occiput-posterior position, low station at application (compared with outlet), larger estimated fetal weight, and arrest or exhaustion as an indication for operative vaginal delivery (compared with a fetal indication) were significantly associated with a failed operative vaginal delivery. In multivariable analysis, factors that remained independently associated with operative vaginal delivery failure were race-ethnicity, arrest or exhaustion as an indication for operative vaginal delivery, occiput-posterior position, and a low pelvic application. The area under the curve for this regression was 0.74 (95% confidence interval 0.69-0.77) demonstrating less than optimal prediction of operative vaginal delivery failure. CONCLUSION: Risk factors identified before an operative vaginal delivery attempt cannot be used to accurately predict whether an operative vaginal delivery attempt will fail.