Women who are overweight during pregnancy may be putting their unborn children at risk of congenital heart defects, new government research shows.
Compared with women of normal weight at the beginning of pregnancy, overweight mothers had an 18 percent increased risk of giving birth to a child with certain heart defects, while the most severely obese had a 30 percent increased risk, according to Dr. Sonja Rasmussen, of the CDC's National Center on Birth Defects and Developmental Disabilities.
She and colleagues reported their findings online in the American Journal of Obstetrics and Gynecology.
"Since obesity is a common condition among women of child-bearing age and is increasing in the U.S. in prevalence, it's important for us to understand the impact obesity has on the risk for [all] birth defects," Rasmussen said.
She added that the findings "provide just one more reason why women should maintain a healthy weight … not only because of the impact on their own health, but because of the impact on their baby's health."
Previous research has not been consistent on the relationship between obesity and congenital heart defects, the researchers said.
So they looked at data from the National Birth Defects Prevention Study involving 6,440 infants with congenital heart defects and 5,673 infants without birth defects. The infants' mothers were interviewed between 1997 and 2004.
In general, mothers of infants with heart defects reported significantly more smoking, gestational diabetes, and hypertension during pregnancy than mothers of healthy babies.
The researchers found that overall, a pre-pregnancy body mass index (BMI) that was above normal was associated with an increased risk for several congenital heart defects.
When comparing by different BMI categories, overweight women had a 16 percent increased risk of having children with heart defects -- particularly right ventricular outflow tract defects, pulmonary valve stenosis, septal defects, and secundum atrial septal defects.
Obese women had a similar 15 percent increased risk. Their children were likely to have total anomalous pulmonary venous return problems as well as left ventricular outflow tract defects, particularly hypoplastic left heart syndrome.
Severely obese women had the highest risk, at 31 percent. The most common defects were similar to those for overweight, but also included conotruncal defects and tetralogy of Fallot.
Rasmussen said the relationship between obesity and heart defects may be explained by differences in diet between obese women and normal-weight women.
Another possibility is the influence of diabetes on birth defects, which has been acknowledged in the literature, she said.
They tried to control for diabetes in their study by excluding women who reported having the condition prior to becoming pregnant. However, in their analyses, they found that heavy women who had gestational diabetes had an even greater risk for congenital heart defects than heavy women who didn't develop the condition (1.46 versus 1.17, respectively, for overweight women and 1.82 versus 1.17, respectively for obese women).
"Women who had gestational diabetes were at even higher increased risk, and that's one of the things that makes us suspicious," Rasmussen said. "Gestational diabetes is something that occurs later in pregnancy, after the heart's already formed. So it makes us wonder whether some of those women [had diabetes] before they became pregnant, and it was just called gestational diabetes because it was diagnosed during pregnancy."
Although the study may have been limited by bias from self-reported data, Rasmussen said it calls for further research into the relationship between obesity and diabetes, and that women should be wary of its implications.
"This doesn't mean that women should start dieting during pregnancy," she said. "These defects occur very early in pregnancy. So this means they should try to be a healthy weight before that."