A risk model for intrapartum cesarean delivery in women with BMI < 18.5

To identify factors associated with intrapartum cesarean delivery (CD) among underweight women undergoing a trial of labor, and to evaluate whether underweight BMI independently influences intrapartum cesarean risk.

A retrospective cohort study conducted at a single university-affiliated tertiary medical center, including women with a pre-pregnancy BMI < 18.5 kg/m² who planned a trial of labor between 2012 and 2024. Women undergoing elective CD or with nonviable fetuses were excluded. Associations were assessed using multivariable logistic regression. To evaluate the independent effect of BMI, an adjusted analysis was performed on the combined underweight and normal-BMI cohort (n = 77,662). Predictive performance was evaluated using receiver operating characteristic analysis, calibration testing, and decision-curve analysis.

Among 147,045 deliveries, 8,967 women (6.1%) had a BMI < 18.5 kg/m² and attempted a trial of labor; 287 (3.2%) underwent intrapartum CD, significantly lower than in a concurrent normal-BMI cohort (4.3%, p < 0.001). In the combined cohort analysis, underweight BMI was independently associated with a lower risk of intrapartum CD (aOR 0.71, 95% CI 0.63-0.81, p < 0.001), with no significant interaction between BMI category and any identified risk factor. Seven factors were independently associated with intrapartum CD: previous CD (aOR 14.57, 95% CI 8.60-24.69), multiple gestation (aOR 8.69, 95% CI 5.16-14.64), nulliparity (aOR 7.39, 95% CI 5.05-10.81), preeclampsia (aOR 3.76, 95% CI 1.96-7.21), maternal age ≥ 40 years (aOR 2.98, 95% CI 1.90-4.68), maternal height < 160 cm (aOR 2.19, 95% CI 1.68-2.84), and induction of labor (aOR 1.66, 95% CI 1.20-2.29). The model demonstrated good discrimination (AUC 0.746, 95% bootstrap CI 0.722-0.768), with sensitivity 92.7%, PPV 6.2%, and NPV 99.1%.

Underweight women have a high likelihood of successful vaginal birth. Underweight BMI was independently associated with a lower risk of intrapartum CD, with no effect modification by any established risk factor. Intrapartum cesarean risk in this population is driven by established obstetric factors rather than low BMI itself, supporting individualized, risk-based counseling and management.