Higher maternal body mass index is associated with increased oxytocin requirements during induction of labor: a population-based cohort study.

BACKGROUND: Maternal obesity is associated with increased risks of gestational diabetes, gestational hypertension, preeclampsia, and post-term pregnancy, all of which increase the likelihood of induction of labor. Current guidelines for oxytocin dosing and administration do not account for maternal body mass index (BMI). OBJECTIVE: To evaluate the association between maternal BMI and (1) cumulative oxytocin dose and (2) maximum oxytocin infusion rate during the first stage of labor among nulliparous women undergoing term induction of labor. STUDY DESIGN: This prospective, population-based cohort study included 1,163 nulliparous women with singleton term pregnancies undergoing induction of labor with oxytocin between 2017 and 2019 in Sweden. Cumulative oxytocin dose, from initiation of infusion until birth, was available for 637 women, and maximum oxytocin infusion rate during the first stage of labor for 974 women. Participants were categorized according to WHO BMI classification: normal weight (<25 kg/m2, n=527), overweight (25.0-29.9 kg/m2, n=359), obesity class I (30.0-34.9 kg/m2, n=167) and obesity class II-III (≥ 35.0 kg/m2, n=108). Kaplan-Meier analyses and Cox proportional hazard models were used to assess the probability of vaginal birth or the decision to perform an emergency cesarean section at a given cumulative oxytocin dose/maximum oxytocin infusion rate, adjusting for maternal age. RESULTS: Mean cumulative oxytocin dose increased progressively across BMI categories (normal weight: 6,230 mU; overweight: 7,410 mU; obesity class I: 8,698 mU; obesity class II-III 10,128 mU; p<0.001). Maximum oxytocin infusion rate also increased with BMI (normal weight: 17.2 mU/min; overweight 18.8 mU/min; obesity class I; 21.1 mU/min and obesity class II-III 24.1 mU/min; p<0.001). Kaplan-Meier curves demonstrated that women with higher BMI required progressively higher cumulative oxytocin doses and maximum oxytocin infusion rates to give birth (vaginal birth or emergency cesarean section) (log-rank P < 0.001). This association remained after adjustment for potential confounders in Cox proportional hazards models. The adjusted Hazard Ratio (aHR) 95% confidence interval (CI)) for birth at a given cumulative oxytocin dose =0.84 (0.70-1.01) for overweight, aHR (95% CI) =0.73 (0.57-0.92) for obesity class I and aHR (95% CI) 0.63 (0.48-0.84) for obesity class II-III). CONCLUSION: Increasing maternal BMI was independently associated with higher cumulative oxytocin doses and infusion rates during IOL. These findings suggest reduced oxytocin responsiveness with increasing BMI and support consideration of BMI tailored dosing strategies during labor induction.