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

To determine risk factors for cesarean delivery in nulliparas at labor admission.

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.

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 (CI): 4.5 to 5.7; reference ⩾5 cm) and BMI ⩾40 kg m(-2) (OR 5.1, 95% CI: 4.6 to 5.7; reference BMI <25.0 kg m(-2)) had the highest odds for cesarean delivery.

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.