By David Morgan

WASHINGTON (Reuters) - Women who are overweight or obese when they get pregnant are more likely to give birth to children with congenital heart defects, according to a U.S. government study released on Thursday.

The study, conducted by the U.S. Centers for Disease Control and Prevention, concluded that women who were overweight or obese at the time they became pregnant were 18 percent more likely to give birth to babies with heart defects, while severely obese women had a 30 percent increased risk.

The babies had problems including obstructive defects on the right side of the heart and defects in the tissue separating the heart's two upper chambers, the researchers reported in the American Journal of Obstetrics and Gynecology.

"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," said Dr. Edwin Trevathan, a CDC expert on birth defects and developmental disabilities.

The CDC, the U.S. government's disease watchdog, recommends that overweight women work with their doctors to achieve a healthy weight before pregnancy.

Researchers examined the health of 6,440 infants with congenital heart defects and 5,673 infants without problems, all of whose mothers were interviewed as part of the CDC's National Birth Defects Prevention Study.

They assessed obesity according to each woman's body mass index, or BMI, which relates weight to height. A woman 5 feet, 5 inches tall and weighing 190 pounds (86 kg) would have a BMI of 31.6, while a woman of the same height who weighs 160 pounds (72 kg) would have a BMI of 26.6.

The researchers defined overweight as a BMI of 25 to 29.9, moderate obesity as a BMI of 30 to 34.9 and severe obesity as a BMI of 35 or above.

The results showed links between obesity and 10 out of 25 kinds of heart defects. Five of the 10 were associated with women who were overweight before pregnancy.

The study accounted for important factors such as maternal age and race-ethnicity. Mothers who had diabetes before pregnancy, a strong risk factor for heart defects, were excluded from the research.

(Editing by Maggie Fox and Xavier Briand)

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