Population risk factors for nulliparous, term, singleton, vertex caesarean birth: a national cross-sectional study

Objective To characterise medical, obstetric and demographic risk factors associated with nulliparous, term, singleton, vertex (NTSV) caesarean birth. Study design Cross-sectional study. Setting United States delivery hospitalisations. Population NTSV births in 2016-18 US natality data. Methods This study analysed a national sample of natality data generated by the United States National Vital Statistics System. NTSV deliveries were identified. The primary outcome was caesarean birth. Risk factors including maternal age, body mass index (BMI) and pregestational diabetes were analysed. Multivariable log-linear regression models analysed factors associated with NTSV caesarean with adjusted risk ratios (aRR) as measures of effect. Results Of 11 622 400 deliveries, 3 764 707 met NTSV criteria, and their caesarean section rate was 25.9%. Maternal age 35-39 years (aRR 1.51, 95% CI 1.50-1.52) and 40-54 years (aRR 2.03, 95% 2.00-2.05) compared with age 19-34 years; BMI 25 to <30 kg/m2 (aRR 1.32, 95% CI 1.31-1.33), 30 to <35 kg/m2 (aRR 1.57 95% CI 1.56-1.58), 35 to <40 kg/m2 (aRR 1.82, 95% CI 1.80-1.83) and >= 40 kg/m2 (aRR 2.17, 95% CI 2.15-2.19) compared with BMI 18.5-24.9 kg/m2; and pregestational diabetes (aRR 1.54, 95% CI 1.51-1.57) were all associated with increased risk. Risk factors allowed stratification of patients into high-risk versus low-risk groups. The NTSV caesarean rate was 37.9% in women who had one or more of the following characteristics: age >= 35 years, BMI >= 30 kg/m2 or pregestational diabetes. In comparison, the NTSV caesarean rate was 20.8% among women without any of these three risk factors (P < 0.01). Conclusion Among NTSV births, BMI, maternal age and medical conditions are important risk factors for caesarean delivery.