OBJECTIVE: To identify maternal clinical risk factors for postcesarean maternal infection in a randomized clinical trial of preincision extended-spectrum antibiotic prophylaxis. METHODS: We conducted a planned secondary analysis of a randomized clinical trial. Patients were 24 weeks of gestation or greater and delivered by cesarean after a minimum of 4 hours of ruptured membranes or labor. All participants received standard preincision prophylaxis and were randomized to receive azithromycin or placebo. The primary outcome for this analysis is maternal infection: a composite outcome of endometritis, wound infection (superficial or deep), or other infections occurring up to 6 weeks postpartum. Maternal clinical characteristics associated with maternal infection, after controlling for azithromycin assignment, were identified. These maternal factors were included in a multivariable logistic regression model for maternal infection. RESULTS: Of 2,013 patients, 1,019 were randomized to azithromycin. Overall, 177 (8.8%) had postcesarean maternal infection. In the final adjusted model, compared with the reference groups, women of black race-ethnicity, with a nontransverse uterine incision, with duration of membrane rupture greater than 6 hours, and surgery duration greater than 49 minutes, were associated higher odds of maternal infection (all with adjusted odds ratios [ORs] of approximately 2); azithromycin was associated with lower odds of maternal infection (adjusted OR 0.4, 95% confidence interval 0.3-0.6). CONCLUSION: Despite preincision azithromycin-based extended-spectrum antibiotic prophylaxis, postcesarean maternal infection remains a significant source of morbidity. Recognition of risk factors may help guide innovative prevention strategies.