Maternal body mass index and risk of obstetric, maternal and neonatal outcomes: A cohort study of nulliparous women with spontaneous onset of labor.

Abstract

This study investigates associations between maternal body mass index (BMI) early in pregnancy and obstetric interventions, maternal and neonatal outcomes. This is a cohort study of nulliparous women originally included in a cluster randomized controlled trial carried out at 14 Norwegian obstetric units between 2014 and 2017. The sample included 7189 nulliparous women with a singleton fetus, cephalic presentation and spontaneous onset of labor at term, denoted as group 1 in the Ten-Group Classification System. The women were grouped according to the World Health Organization BMI classifications: underweight (BMI<18.5), normal weight (BMI 18.5-24.9), pre-obesity (BMI 25.0-29.9), obesity class I (BMI 30.0-34.9), and obesity classes II and III (BMI≥35.0). We used binary logistic regression to estimate crude and adjusted odds ratios (ORs) of the interventions and outcomes, with associated 95% confidence intervals (CIs), comparing women in different BMI groups with women of normal weight. We found an increased risk of intrapartum cesarean section in women of obesity class I and obesity classes II and III, with adjusted OR of 1.70 (95% CI 1.21-2.38) and 2.31 (95% CI 1.41-3.77), respectively. Women in obesity groups had a gradient of risk of epidural analgesia and use of continuous CTG (including STAN), with adjusted OR of 2.39 (95% CI 1.69-3.38) and 3.28 (95% CI 1.97-5.48), respectively. Women in obesity classes II and III had higher risk of amniotomy (adjusted OR=1.42, 95% CI 1.02-1.96), oxytocin augmentation (adjusted OR=1.54, 95% CI 1.11-2.15), obstetric anal sphincter injuries (adjusted OR=2.21, 95% CI 1.01-4.85) and postpartum hemorrhage≥1000mL (adjusted OR=2.20, 95% CI 1.29-3.78). We found a reduced likelihood of spontaneous vaginal delivery for pre-obese women (adjusted OR=0.85, 95% CI 0.74-0.97) and no associations between maternal BMI and neonatal outcomes. Obese women in Ten-Group Classification System group 1 had increased risks of obstetric interventions and maternal complications. There was a gradient of risk for intrapartum cesarean section, with the highest risk for women in obesity classes II and III. No associations between maternal BMI and neonatal outcomes were observed.