Delivery timing strategies in NTSV pregnancies associated with varying cesarean rates across maternal risk strata.

Objective: To externally validate a risk stratified delivery timing model for nulliparous, term, singleton, vertex (NTSV) cesarean reduction using national data. Design: Population based cohort study of NTSV births in US National Vital Statistics System (NVSS) natality files, 2020 to2024, using logistic regression for cesarean predictors and risk stratified Monte Carlo simulation (10,000 iterations per strategy and risk group) to evaluate delivery timing policies. Setting: All live births in the US recorded in the NVSS natality files. Participants: NTSV patients with term (37+ weeks) pregnancies and complete gestational age and delivery mode data (N=5 776 412). A sensitivity cohort excluded pre 39 week deliveries and pregnancies with preexisting diabetes or hypertension. Exposures: Delivery timing strategies defined by gestational age and labor onset (elective induction at 39, 40, or 41 weeks, or expectant management to 42 weeks), evaluated within maternal age and body mass index (BMI) risk strata (low: age <35 and BMI <30; moderate: age > 35 or BMI > 30; high: age > 35 and BMI > 35). Main Outcomes and Measures: Primary outcome was cesarean delivery, measured as the proportion of deliveries completed by cesarean across gestational ages, labor onset types, and age BMI strata. Secondary outcomes included gestational age specific cesarean rates, area under the receiver operating characteristic curve (AUC) for cesarean prediction, and simulated mean cesarean rates with 95% simulation intervals under four delivery timing strategies within each risk group. Results: The overall NTSV cesarean rate was 26.4%. Cesarean Rates were U shaped across gestational ages, with the lowest rate at 38 weeks (24.9%) and higher rates at 37 weeks (29.8%) and 41 to 42 weeks (28.1 to 28.5%). Risk group distribution was 64.9% low, 33.7% moderate, and 1.4% high. Model AUC was 0.65. Induction had higher cesarean rates than spontaneous labor (29.3% vs 24.2%; odds ratio 1.30, 95% confidence interval 1.29 to 1.30). Monte Carlo simulation favored induction at 39 weeks for high risk patients (59.3%) and expectant management to 41 to 42 weeks for low risk patients (19.1%). Conclusions and Relevance: A risk stratified NTSV labor management model showed external validity in 5.8 million US births and consistently identified risk-specific timing strategies that lowered cesarean rates, supporting individualized delivery timing policies.