The links between weight and other defects, including debilitating heart problems, have been less well established, the researchers said.

Compared with normal-weight women, those who were obese or overweight before pregnancy faced double the risk of having babies with heart defects and double the risk of multiple birth defects, the study said.

"This is yet another adverse health outcome associated with overweight and obesity and people need to know that," said CDC epidemiologist Margaret Watkins. "Obesity prevention efforts are needed to increase the number of women who are at a healthy weight before they become pregnant."

Watkins said normally 3 to 5 percent of infants are born with major birth defects.

Obese women faced an even higher risk — more than triple that of normal-sized women — of having babies with a defect known as omphalocele, in which intestines or other abdominal organs protrude through the navel.

The study was released Monday in May's editions of the journal Pediatrics.

Reasons for the links are uncertain but may include nutritional deficits in women with poor eating habits or diabetes, which is common in obesity and is known to increase risks for birth defects, according to the research team led by Watkins.

It also may be that obese women have increased but sometimes unmet needs for nutrients such as folic acid that can protect against some birth defects, the researchers said.

"Although the biological mechanism(s) behind obesity and birth defects is unknown, efforts to ensure that reproductive-aged women are of healthy weight before pregnancy should not await the elucidation of the mechanisms," they wrote.

The authors examined data from births in a five-county area of metropolitan Atlanta between January 1993 and August 1997. Researchers studied 645 infants with birth defects and 330 without in a case-controlled study.

Mothers were considered overweight if they had a body-mass index between 25 and 30, and obese if their BMI, a height-weight ratio, was 30 or higher.

Dr. Richard J. Deckelbaum, director of Columbia University's Institute of Human Nutrition, said being overweight and obese are reversible risk factors that can lead to better outcomes for babies and mothers.

"It's an underappreciated link between overweight and obesity before pregnancy and outcomes for the infant and even for the mother during pregnancy," Deckelbaum said