Title : A simple bedside composite risk score (AB-100 rule) for predicting cesarean birth in nulliparous women: A historical cohort study with external validation
Abstract:
Background: Cesarean birth rates continue to rise worldwide. Advanced maternal age and elevated Body Mass Index (BMI) are well-established risk factors for intrapartum cesarean delivery in nulliparous women. However, translating these epidemiological risk factors into a simple clinical tool that can support individualized counseling and shared decision-making remains challenging. This study aimed to develop and externally validate a practical bedside composite risk score integrating maternal age and BMI (the AB-100 rule) for predicting cesarean birth.
Methods: We conducted a historical cohort study including nulliparous women with singleton term pregnancies undergoing trial of labor at a tertiary academic center. A composite score was calculated as maternal age (years) + 2 × BMI at delivery (kg/m²) (AB score). Logistic regression and receiver operating characteristic (ROC) curve analysis were used to assess predictive performance. A clinically practical threshold of AB score ≥100 was evaluated. External validation was performed using an independent cohort managed by multiple obstetricians. Decision Curve Analysis (DCA) was conducted to assess potential clinical utility.
Results: Higher composite scores were significantly associated with increased risk of cesarean birth. Women with AB score ≥ 100 had a markedly higher cesarean rate compared with those with lower scores (67.6% vs 17.3%). The model demonstrated moderate discrimination and consistent performance in the external validation cohort. Decision curve analysis indicated a positive net benefit across clinically relevant threshold probabilities.
Conclusions: Although maternal age and BMI are known determinants of cesarean birth risk, integrating them into a simple bedside composite score provides an intuitive framework for early risk communication. The AB-100 rule may support shared decision-making and individualized delivery planning in nulliparous women.
