Body Mass Index and the odds of reaching the active phase of induced labour: A cohort study

Objective: To investigate the association between Body Mass Index (BMI) and the odds of reaching the active phase of induced labour. Design: Cohort study. Setting: Delivery sites in the Central Denmark Region from 2013 to 2022. Population Pregnant women with a singleton foetus in cephalic presentation induced at 37 weeks of gestation or beyond. We excluded women with spontaneous pre-labour rupture of membranes, uterine scar or intrauterine foetal demise. Methods: Information was retrieved from the electronic patient record. We used logistic regression analyses adjusting for potential confounders and modelled BMI as categorical and continuous variable using restricted cubic spline analysis. Exposure was defined according to the World Health Organization BMI groups. Main Outcome Measure Reaching active phase of labour. Results: Of the 22,114 women in the cohort, the proportion reaching the active phase of labour was 98%, 97%, 96%, 95% and 95%, for each BMI group. Compared to normal weight, the adjusted odds ratios (aOR) and 95% confidence intervals (CI) were aOR 0.6 (95% CI 0.5, 0.8) for overweight, aOR 0.5 (95% CI 0.3, 0.7) for obesity class I, aOR 0.4 (95% CI 0.3, 0.5) for obesity class II and aOR 0.4 (95% CI 0.2, 0.7) for obesity class III. A similar pattern was found when using restricted cubic splines. Conclusion: In singleton pregnant women induced at term, higher BMI was associated with lower odds of reaching the active phase of labour. Regardless of BMI, 95% to 98% of all women in the cohort reached the active phase of labour, and 80% to 90% gave birth vaginally.