Association of maternal body mass index with success and outcomes of attempted operative vaginal delivery.

BACKGROUND: Increasing maternal body mass index (BMI) is associated with increased morbidity at cesarean delivery in a dose-dependent manner. In some clinical scenarios, operative vaginal delivery is a strategy to prevent the morbidity associated with second stage cesarean delivery, but the relationship between maternal BMI and outcomes of attempted operative vaginal delivery is not well characterized.

OBJECTIVE: To assess whether success of and adverse outcomes after attempted operative vaginal delivery are associated with maternal BMI at delivery among nulliparous individuals.

STUDY DESIGN: This is a secondary analysis from the prospective cohort Nulliparous Pregnancy Outcomes Study: Monitoring Mothers-To-Be study. This analysis included cephalic liveborn non-anomalous singleton pregnancies ≥34 weeks at delivery with an attempted operative vaginal delivery (either forceps or vacuum). The primary exposure was maternal BMI at delivery (BMI ≥30 kg/m2 vs BMI <30 kg/m2 [referent]). The primary outcome was an unsuccessful operative vaginal delivery attempt, defined as a cesarean delivery following an attempted operative vaginal delivery. Secondary outcomes included maternal and neonatal adverse outcomes. Multivariable logistic regression was used, and statistical interaction between operative instrument type (vacuum vs forceps) and BMI was assessed.

RESULTS: Of 10,038 assessed individuals, 791 (7.9%) had an attempted operative vaginal delivery and were included in this analysis. Forty-one percent (n=325) had a BMI ≥30 kg/m2 at delivery. Overall, 5% (42/791) of participants experienced an unsuccessful operative vaginal delivery. Individuals with a BMI ≥30 kg/m2 at delivery were more than twice as likely to have an unsuccessful operative vaginal delivery compared with those with a BMI <30 kg/m2 (8.0 vs 3.4%; p=0.005; adjusted odds ratio (aOR): 2.23, 95% confidence interval (CI) 1.16 to 4.28). Composite maternal morbidity and composite neonatal morbidity did not vary by BMI group. There was no evidence of interaction or effect modification by operative instrument type for rate of unsuccessful operative vaginal delivery attempt, composite maternal morbidity, or composite neonatal morbidity.

CONCLUSION: Among nulliparous individuals who underwent an attempted operative vaginal delivery, those with a BMI ≥30 kg/m2 at delivery were more likely to have an unsuccessful operative vaginal delivery attempt compared with those with a BMI <30 kg/m2 . There were no differences in composite maternal or neonatal morbidity after attempted operative vaginal delivery by BMI category.