Association of cesarean delivery risk with obesity and gestational diabetes mellitus: a cross-sectional study of NHANES 2005–2018

This study aimed to investigate the independent and synergistic associations of obesity and gestational diabetes mellitus (GDM) with cesarean delivery risk, while comparing body mass index (BMI) and body roundness index (BRI) as indicators of obesity in the National Health and Nutrition Examination Survey (NHANES). We analyzed cross-sectional data from 6292 women aged ≥20 years in NHANES 2005–2018. Cesarean delivery history and GDM status were self-reported. Obesity was defined as BMI ≥ 30 kg/m2 or BRI > 5.5. Weighted multivariable logistic regression models adjusted for demographics (age, race, education, poverty-income ratio), smoking, and alcohol use were used to estimate odds ratios (ORs) and 95% confidence intervals (CIs). Restricted cubic spline and subgroup analyses evaluated nonlinear relationships and effect modifications. All analyses were performed using R 4.5.1. Overall, 47.1% reported cesarean delivery. After full adjustment, obesity (BMI-based: aOR = 1.11, 95% CI: 1.06–1.17; BRI-based: aOR = 1.10, 95% CI: 1.05–1.14) and GDM (aOR = 1.09, 95% CI: 1.03–1.16) independently increased cesarean risk. A significant synergistic effect was observed: women with both obesity and GDM had 24% higher risk (BMI-based aOR = 1.24, 95% CI: 1.15–1.34; BRI-based aOR = 1.22, 95% CI: 1.14–1.30) compared to those without either condition. Subgroup analyses revealed that the obesity-related effect was more significant only among women with GDM. BRI demonstrated greater stability than BMI, showing no interaction with smoking history. Both obesity and GDM independently elevate cesarean delivery risk, with a pronounced synergistic effect when coexisting. BRI performs comparably to BMI but exhibits superior stability in risk prediction. Integrated prenatal screening for metabolic risks and targeted weight management may reduce avoidable cesarean deliveries.