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

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 N orwegian M other and C hild C ohort ( M o B a), 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.