Obesity and the risk of stillbirth: a population-based cohort study

OBJECTIVE: Obesity is a known risk factor for stillbirth. However, this relationship has not been characterized fully. We attempted to further examine this relationship with a focus on delivery near and at term. STUDY DESIGN: We designed a retrospective cohort study of singleton nonanomalous live births and stillbirths in the states of Washington and Texas to examine the associations of maternal prepregnancy body mass index (BMI) and risk of stillbirth. Confounder-adjusted hazard ratio of stillbirth in relation to BMI was estimated through Cox proportional hazards regression model. The hazard ratio was used to estimate the population-attributable risk. We also estimated the fetuses who were at risk for stillbirth based on gestational age. RESULTS: Among 2,868,482 singleton births, the overall stillbirth risk was 3.1 per 1000 births (n = 9030). Compared with normal-weight women, the hazard ratio for stillbirth was 1.36 for overweight women, 1.71 for class I obese women, 2.00 for class II obese women, 2.48 for class III obese women, and 3.16 for women with a BMI of >= 50 kg/m(2). The fetuses who are at risk for stillbirth increased after 39 weeks' gestation for each obesity class; however, the risk increased more rapidly with increasing BMI. Women with a BMI of >= 50 kg/m(2) were at 5.7 times greater risk than normal weight women at 39 weeks' gestation and 13.6 times greater at 41 weeks' gestation. Obesity was associated with nearly 25% of stillbirth that occurred between 37 and 42 weeks' gestation. CONCLUSION: There is a pronounced increase in the risk of stillbirth with increasing BMI; the association is strongest at early-and late-term gestation periods. Extreme maternal obesity is a significant risk factor for stillbirth.