Women who are obese before becoming pregnant seem to have a higher risk of stillbirth and of having an infant die soon after birth, according to a new study.

The study of more than 24,000 Danish women found that, compared with normal-weight women, those who were obese before pregnancy had twice the risk of stillbirth and to have a baby die within a month, according to findings published in the British Journal of Gynecology.

For all of the women, regardless of weight, stillbirths and newborn deaths were relatively uncommon. Among obese women, stillbirths occurred at a rate of less than 12 per 1,000 births, and the rate of newborn death was lower still, at about 7 per 1,000 births.

But the figures compare with a stillbirth rate of 4 per 1,000 births among normal-weight women, and a newborn death rate of less than 3 for every 1,000 births.

Women who were overweight but not obese had risks comparable to those of normal-weight women, according to the researchers, led by Dr. Janni Kristensen of Aarhus University Hospital in Denmark.

“Our data suggest that Obesity is associated with a marked increased risk of stillbirth and neonatal deaths,” the study authors write, “emphasizing the need for public health interventions to prevent obesity in young women.”

The study is not the first to link obesity to greater odds of stillbirth and newborn death. In the past, researchers have found that the risks may be partially explained by the higher rates of diabetes and high blood pressure seen in overweight pregnant women.

But in this study, Diabetes and high blood pressure were not key factors, according to Kristensen’s team. When the researchers excluded women with either of the two disorders from their analysis, the risks of stillbirth and newborn death linked to obesity remained essentially unchanged.

Overall, the investigators found, no single cause of stillbirth or newborn death explained the higher rates among obese women. Compared with their normal-weight and overweight counterparts, obese women had a higher rate of stillbirths classified as “unexplained,” and problems with the placenta -which transfers oxygen and nutrients from the mother’s blood to the fetus -were more common.

Exactly why obesity may raise the risk of stillbirth and early infant death is not clear, but the Danish researchers point out that obesity is known to affect the hormonal system and the metabolism of blood fats. High Cholesterol, they note, may affect certain substances in the body that control blood clotting and blood vessel dilation, and thereby impair blood flow to the placenta.

While the ultimate goal is to prevent obesity in the first place, the researchers add that fetal and newborn deaths could potentially be reduced by referring more obese women to obstetricians who specialize in high-risk pregnancies.

SOURCE: British Journal of Gynecology, April 2005.