More than half of women in the United States who are 20 to 39 years of age are overweight or obese. A similar but somewhat delayed pattern is seen in Europe. Prepregnancy obesity—defined as a body mass index (BMI) of 30 kg/m2 or higher—has been associated with severe complications of pregnancy and delivery, including stillbirth, which accounts for more than half of all perinatal deaths. The investigators used data from the Danish National Birth Cohort of 54,505 pregnant women to examine the association between a high prepregnancy BMI and fetal death while controlling for several parameters. The incidence of obesity in this population was 8%. Another 19% of women were overweight (BMI of 25–29.9 kg/m2), 68% had normal body weight (BMI 18.5–25.0 kg/m2), and 5% were underweight (BMI less than 18.5 kg/m2).
The overall risk of stillbirth in this study was 2.8 per 1000. Compared with women of normal weight, those who were obese or overweight were slightly younger, smoked more, drank less alcohol, and exercised less. Preeclampsia, other hypertensive conditions, and gestational diabetes all were more frequent in obese and overweight women. The relative risk of fetal death rose with gestational age in these groups. After 14 weeks gestation, the expected drop in fetal deaths across gestation was more marked in normal-weight women than in the obese and overweight groups. The adjusted relative risk of stillbirth in obese women was 360% higher when pregnancy lasted more than 280 days. Neither parity nor smoking altered the apparent effect of BMI on fetal death. Adjusted risk estimates changed only minimally after excluding women with preeclampsia, other hypertensive disorders, or diabetes. Overweight women still had twice the risk of stillbirth compared with normal-weight women, and the risk increased 240% for obese women. In addition, obese and overweight women were relatively likely to have unexplained intrauterine deaths and deaths resulting from placental dysfunction after adjusting for possible confounding factors. In this large population of pregnant women, those who were obese before pregnancy were likelier than others to experience fetal death as gestation advanced. Placental dysfunction may be partly responsible. It seems advisable to encourage obese women who want to become pregnant to lose weight.