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

Abstract

Introduction: This study investigates associations between maternal body mass index (BMI) early in pregnancy and obstetric interventions, maternal and neonatal outcomes.

Material and methods: 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.

Results: 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 ≥1000 mL (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.

Conclusions: 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.

Keywords: Ten-Group Classification System; body mass index; cesarean section; maternal outcome; neonatal outcome; nulliparous; obstetric interventions; postpartum hemorrhage.