The impact of body mass index on maternal and neonatal outcomes: a retrospective study in a UK obstetric population, 2004-2011

Objective: To assess the prevalence of overweight and obesity, and the impact of body mass index (BMI) on maternal and neonatal outcomes, in a UK obstetric population.

Design: Retrospective study.

Setting: A tertiary referral unit in Northern Ireland.

Population: A total of 30 298 singleton pregnancies over an 8-year period, 2004-2011.

Methods: Women were categorised according to World Health Organization classification: underweight (BMI < 18.50 kg/m(2)); normal weight (BMI 18.50-24.99 kg/m(2); reference group); overweight (BMI 25.00-29.99 kg/m(2)); obese class I (BMI 30.00-34.99 kg/m(2)); obese class II (BMI 35-39.99 kg/m(2)); and obese class III (BMI ≥ 40 kg/m(2)). Maternal and neonatal outcomes were examined using logistic regression, adjusted for confounding variables.

Main outcome measures: Maternal and neonatal outcomes.

Results: Compared with women of normal weight, women who were overweight or obese class I were at significantly increased risk of hypertensive disorders of pregnancy (OR 1.9, 99% CI 1.7-2.3; OR 3.5, 99% CI 2.9-4.2); gestational diabetes mellitus (OR 1.7, 99% CI 1.3-2.3; OR 3.7, 99% CI 2.8-5.0); induction of labour (OR 1.2, 99% CI 1.1-1.3; OR 1.3, 99% CI 1.2-1.5); caesarean section (OR 1.4, 99% CI 1.3-1.5; OR 1.8, 99% CI 1.6-2.0); postpartum haemorrhage (OR 1.4, 99% CI 1.3-1.5; OR 1.8, 1.6-2.0); and macrosomia (OR 1.5, 99% CI 1.3-1.6; OR 1.9, 99% CI 1.6-2.2), with the risks increasing for obese classes II and III. Women in obese class III were at increased risk of preterm delivery (OR 1.6, 99% CI 1.1-2.5), stillbirth (OR 3.0, 99% CI 1.0-9.3), postnatal stay > 5 days (OR 2.1, 99% CI 1.5-3.1), and infant requiring admission to a neonatal unit (OR 1.6, 99% CI 1.0-2.6).

Conclusions: By categorising women into overweight and obesity subclassifications (classes I -III), this study clearly demonstrates an increasing risk of adverse outcomes across BMI categories, with women who are overweight also at significant risk.