Adding to the growing concern over the hazards of obesity to pregnant women and their babies, two new studies have found that women who are obese at the time of conception are at least twice as likely to have children with neural tube defects, such as spina bifida, as women of normal weight.
Among the heaviest women, the risk rose to four times normal, according to the reports in today's Journal of the American Medical Assn. The studies represent the first time a specific birth defect has been definitively linked to a mother's weight, according to Dr. James Mills of the National Institute of Child Health and Human Development.
Researchers from the California Birth Defects Monitoring Program and Boston University found, furthermore, that folic acid supplements, which normally protect against neural tube defects, are of no benefit in women who are at least 20% overweight.
Neural tube defects are the most common and severe congenital malformations, affecting about six children in every 10,000 in California. Spina bifida, which accounts for more than half of the defects, is an incomplete closure of the spinal column that usually results in paralysis of the legs.
"It was not too surprising to find that there are complications during childbirth in obese women, but it is very surprising to find a specific anatomical defect" in the infant, said Dr. Richard B. Johnston of the March of Dimes Birth Defect Foundation.
Pre-pregnancy counseling has historically focused on the health hazards of underweight mothers, such as premature, underweight infants, he said. "These two studies tell us we also need to focus on obese women and try to get their weight down to a level that is closer to ideal."
"This expands the very short list of metabolic factors that cause birth defects," Mills said. The other two are diabetes and folic acid deficiencies. Otherwise, most birth defects are genetic in origin.
Experts are at a loss to explain how maternal obesity causes the defects, however. "We do know that a link exists," said epidemiologist Gary M. Shaw of the California program. "Right now we don't know why. It could be something nutritional that we were unable to measure; it could be something metabolic in how women handle nutrient intake."
