Mortality in infants of obese mothers: is risk modified by mode of delivery?

  Objective. To examine the association between maternal obesity and infant mortality, while including information about mode of delivery and interpregnancy weight change. Design. Register‐based cohort study. Setting and population. A total of 1 199 183 singletons, including 3481 infant deaths, from the Swedish Birth Register 1992–2006. Methods. Maternal body mass index (BMI) was obtained from self‐reports in early pregnancy and categorized as underweight (<18.5 kg/m 2 ), normal‐weight (18.5–24.9 kg/m 2 ), overweight (25–29.9 kg/m 2 ), obese (30–34.9 kg/m 2 ) and extremely obese (≥35 kg/m 2 ). Cox regression was used to estimate hazard ratios (95% confidence intervals). Infants of normal‐weight women were the referent. Main outcome measures. Neonatal and infant mortality. Results. Infant mortality increased with increasing maternal fatness [adjusted hazard ratios 1.2 (1.1–1.3), 1.4 (1.2–1.6) and 2.1 (1.8–2.5) for overweight, obesity and extreme obesity, respectively]. When accounting for mode of delivery, neonatal mortality was increased in infants of obese and extremely obese mothers after spontaneous births [adjusted hazard ratios 1.8 (1.4–2.4) and 2.6 (1.8–4.0), respectively, after term births, and 1.4 (1.1–1.9) and 2.2 (1.5–3.3), respectively, after preterm births]. No excess risk was present for infants of obese mothers after induced term and preterm births ( p ‐values for interaction <0.05). For post‐neonatal mortality, no interaction between mode of delivery and maternal obesity was observed. In women with two subsequent pregnancies, high interpregnancy weight change >1 BMI unit (1 kg/m 2 ) seemed to involve a modest increase in neonatal mortality in the second infant, but only after spontaneous births [adjusted odds ratio 1.3 (0.9–1.7)]. Conclusions. Maternal obesity, especially at levels that may involve cardiometabolic morbidity, was associated with increased mortality in the offspring.