Intrapartum cesarean delivery in nulliparas: risk factors compared by two analytical approaches

OBJECTIVE: To determine risk factors for cesarean delivery in nulliparas at labor admission. STUDY DESIGN: Nulliparas with live-born, singleton gestations >= 37 weeks in spontaneous or induced labor were analyzed from the Consortium on Safe Labor database in a retrospective observational study. Classification and regression tree (CART) and multivariate logistic regression analysis determined risk factors for cesarean delivery. RESULT: Of the 66 539 nulliparas, 22% had a cesarean delivery. In the CART analysis, the first cervical dilation exam was the first branch followed by body mass index (BMI). Cesarean deliveries occurred in 45%, 25%, 14% and 10% of deliveries at < 1, 1 to 3,4 and >= 5 cm dilated, respectively. The BMI influence was most evident in the < 1 cm dilation category with 26% of BMI < 25 kg m(-2) and 66% of BMI >= 40 kg m(-2) having a cesarean delivery. The fewest cesarean deliveries (5%) occurred in those >= 5 cm and BMI < 25 kg m(-2). In the multivariate regression analysis, first cervical dilation exam < 1 cm (odds ratio (OR) 5.1, 95% confidence interval (Cl); 4.5 to 5.7; reference >= 5 cm) and BMI 40 kg m(-2) (OR 5.1, 95% Cl: 4.6 to 5.7; reference BMI < 25.0 kg m(-2)) had the highest odds for cesarean delivery. CONCLUSION: Cervical dilation on admission followed by BMI were the two most important risk factors for cesarean delivery identified in both CART and multivariate regression analysis.