Compared with normal-weight women, those who were overweight at the time of conception had twice the risk of giving birth to babies with heart abnormalities, according to findings from the CDC investigation.
Obese women were more than three times as likely to deliver babies with the neural tube defect spina bifida or the abdominal malformation known as omphalocele, in which the intestines or other abdominal organs protrude into the umbilical cord.
While earlier research suggested a link between prepregnancy weight and specific birth defects, the CDC report offers the most comprehensive evidence yet of the negative impact of maternal obesity on the health of unborn babies. The study is published in the May 5 issue of the journal Pediatrics.
"This is yet another adverse health outcome linked to obesity and overweight," CDC epidemiologist Margaret L. Watkins, MPH, tells WebMD. "It emphasizes the need for prevention efforts aimed at getting a higher proportion of women to achieve a healthy weight before they get pregnant."
The CDC study compared women who delivered babies with birth defects in a five-county area of Atlanta between 1993 and 1997 with mothers who gave birth to healthy babies during the same period. Women with pre-existing diabetes were excluded from the study.
The mothers were considered overweight if they had a body mass index (a comparison of weight and height) between 25-30. Women with a BMI of 30 or more were considered obese.
In addition to the individual birth defects, overweight women were twice as likely to have babies with multiple congenital abnormalities.
Watkins says it is not clear why the offspring of overweight women are at increased risk, but one possible explanation may be metabolic abnormalities common in people who carry extra pounds. Obesity is associated with elevated insulin levels, which has previously been shown to be a risk factor for neural tube defects like spina bifida.
"We know that if you are overweight or obese you are more likely to develop type 2 diabetes, and diabetes is a known risk factor for birth defects," Watkins says. "We excluded all known diabetics from the study, but it is possible that some women had unrecognized or sub-clinical diabetes."
According to the March of Dimes, prepregnancy obesity is a major cause of premature births in the United States. In a report published early last year, the organization's Task Force on Nutrition and Optimal Human Development concluded that birth defects, premature births, and other severe health problems were likely to rise in coming years due to the obesity epidemic.
Columbia University professor of nutrition Richard J. Deckelbaum, MD, who led the task force, tells WebMD that neither women nor their doctors have gotten the message that prepregnancy weight control is associated with delivering a healthy baby.
"This is another reason for women to really make an effort to keep their weight under control," he says. "Maintaining a healthy weight is not only good for a woman's personal health, but for that of her future children. Birth defects carry a tremendous cost, not only for the baby and the family, but for society."