The study, published in the advance online edition of the American Journal of Obstetrics and Gynecology, included the mothers of 6,440 babies born with congenital heart defects between 1997 and 2004.
In telephone interviews, the mothers reported their pre-pregnancy height, weight, and various lifestyle and medical factors.
For comparison, the researchers asked the same questions of 5,673 women who had babies during the same time period without heart defects.
By the same comparison, women who were moderately obese before pregnancy (BMI of 30-34.9) were 15% more likely to have a baby born with heart defects.
Women who were severely obese before pregnancy (BMI greater than 35) were 31% more likely to have a baby born with a heart defect.
"Congenital heart defects are the most common types of birth defect, and among all birth defects, they are a leading cause of illness, death, and medical expenditures," Edwin Trevathan, MD, MPH, director of the CDC's National Center on Birth Defects and Disabilities, says in a news release.
"Women who are obese and who are planning a pregnancy could benefit by working with their physicians to achieve a healthy weight before pregnancy," Trevathan says.
The study doesn't prove that the women's extra weight caused birth defects in their babies. Observational studies like this one can show associations, but they don't prove cause and effect.
Also, the women reported their height and weight; they weren't measured. Self reports of weight aren't always accurate, and that could have affected the results.
"These results support previous studies, as well as provide additional evidence, that there is an association between a woman being overweight or obese before pregnancy and certain types of heart defects," Suzanne Gilboa, PhD, an epidemiologist at CDC's National Center on Birth Defects and Developmental Disabilities, says in a news release.
"This provides another reason for women to maintain a healthy weight. In addition to the impact on a woman's own health and known pregnancy complications associated with maternal obesity, the baby's health could be at risk," says Gilboa, who worked on the new study.
Gilboa's study only compares the odds of having a baby with a heart defect; it doesn't show the odds that any given woman, of any size, would have a baby with a heart defect.