Population Risk Factors for Nulliparous, Term, Singleton, Vertex Cesarean Birth: A National Cross-sectional Study.

OBJECTIVE To characterize medical, obstetric, and demographic risk factors associated with nulliparous, term, singleton, vertex (NTSV) cesarean birth. STUDY DESIGN Cross sectional study. SETTING United States delivery hospitalizations. POPULATION NTSV births in 2016-2018 US natality data. METHODS This study analyzed a national sample of natality data generated by the United States National Vital Statistics System. NTSV deliveries were identified. The primary outcome was cesarean birth. Risk factors including maternal age, body mass index (BMI), and pre-gestational diabetes were analyzed. Multivariable log-linear regression models analyzed factors associated with NTSV cesarean with adjusted risk ratios (aRR) as measures of effect. RESULTS Of 3,764,707 of 11,622,400 deliveries that met NTSV criteria the cesarean rate was 25.9%. Maternal age 35-39 (aRR 1.51, 95% CI 1.50-1.52) and 40-54 (aRR 2.03, 95% 2.00-2.05) compared to age 19-34, 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 to 18.5-24.9 kg/m2 , and pre-gestational diabetes (aRR 1.54, 95% CI 1.51-1.57) were all associated with increased risk. Risk factors allowed stratification of patients into high versus low risk groups. The NTSV cesarean rate was 37.9% in women who had ≥1 of the following characteristics: age ≥35 years, BMI ≥30 kg/m2 , or pregestational diabetes. In comparison, the NTSV cesarean rate was 20.8% among of 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 cesarean.