Pre-pregnant body mass index and recreational physical activity: effects on perinatal mortality in a prospective pregnancy cohort

Abstract

Objective: To examine the effect of maternal pre-pregnant body mass index (BMI) and recreational physical activity on perinatal mortality.

Design: A prospective cohort study.

Setting: The Norwegian Mother and Child Cohort (MoBa), 1999-2008.

Population: Singleton pregnancies without congenital anomalies (n = 77 246).

Methods: Pre-pregnant BMI was classified as underweight (<18.5), normal weight (18.5-24.9), overweight (25-29.9), obese (30-34.9) or morbidly obese (BMI ≥ 35). Risk estimates were obtained by logistic regression and adjusted for confounders.

Main outcome measures: Perinatal death (stillbirth ≥ 22 weeks plus early neonatal death 0-7 days after birth).

Results: An increased risk of perinatal death was seen in obese [odds ratio (OR) 2.4, 95% CI (confidence interval) 1.7-3.4] and morbidly obese women (OR 3.3, 95% CI 2.1-5.1) as compared with normal weight women. In the group participating in recreational physical activity during pregnancy, obese women had an OR of 3.2 (95% CI 2.2-4.7) for perinatal death relative to non-obese women. In the non-active group the corresponding OR was 1.8 (95% CI 1.1-2.8) for obese women compared with non-obese women. The difference in perinatal mortality risk related to obesity between the active and non-active groups was statistically significant (P-value for interaction = 0.046, multiplicative model).

Conclusions: Maternal obesity was associated with a two- to three-fold increased risk of perinatal death when compared with normal weight. For women with a BMI <30 the lowest perinatal mortality was seen in those performing recreational physical activity at least once a week.

Keywords: Body mass index; Norwegian Mother and Child Cohort Study (MoBa); obesity; overweight; perinatal death; recreational physical activity.