TUESDAY, Aug. 7 (HealthDay News) — Women who were obese before they became pregnant had a higher risk of having babies with certain birth defects, including missing limbs, malformed hearts and underdeveloped spinal cords, a new study found.

But the researchers cautioned that overweight women planning to get pregnant should try to lose weight sensibly and carefully.

"We would advise women who are obese to try to maintain a healthy weight, engage in moderate exercise and follow a healthy daily diet," said study lead author Kim Waller, associate professor of epidemiology at the University of Texas at Houston's School of Public Health. "Multivitamins both before and after a woman becomes pregnant are very important."

In particular, women are advised to take 400 micrograms of folic acid daily both before pregnancy and during pregnancy. A multivitamin will usually satisfy this recommendation.

And women should not try fad diets.

"We don't want women who are thinking of becoming pregnant or who are pregnant to rush out and go on a crash diet," Waller cautioned. "If you become pregnant, then, sure, maybe try to lose some weight, but do so very, very carefully and maintain a healthy diet while you're doing so."

"You have to be of a healthy weight not only for yourself but also for a healthy pregnancy," added Dr. Jennifer Wu, an obstetrician/gynecologist at Lenox Hill Hospital in New York City. "You want to try to get down to a healthy weight before you even get pregnant in the first place. Pregnancy is not the time to do a crash diet to try to lose weight."

In 2003 and 2004, 51 percent of U.S. women aged 20 to 39 were overweight or obese, putting them at increased risk for chronic diseases, infertility, irregular menstruation and pregnancy complications, according to background information in the study.

Previous research had shown a strong association between pre-pregnancy body mass index (BMI, a ratio of weight to height) and the risk for certain birth defects, particularly anencephaly — a defect in the closure of the neural tube, which forms the brain and spinal cord of the embryo — and spina bifida.

The link between overweight and obesity and other birth defects has been less clear.

According to the U.S. Agency for Healthcare Research and Quality, as many as one in 33 babies born in the United States has a birth defect.

For the new study, the largest of its kind, Waller and her colleagues interviewed 10,249 women in eight states whose babies had been born with birth defects between 1997 and 2002. Information on the women came from the National Birth Defects Prevention Study.

These women were then compared to 4,065 women who had given birth to babies without birth defects during the same time period.

Sixteen birth defects were studied. Of those, mothers of babies with the following seven birth defects were more likely to have been obese than mothers of infants without birth defects:

• Spina bifida, or the incomplete development of the brain, spinal cord and/or meninges (the protective covering around the brain and spinal cord). This is the most common neural tube defect in the United States and affects up to 2,000 of the more than 4 million babies born annually, according to the National Institutes of Health.

• Heart defects.

• Anorectal atresia, or malformation of the anal opening.

• Hypospadias, or an abnormally placed urethral opening in males — on the underside instead of the end of the penis.

• Limb reduction defects, such as small or missing toes, fingers, arms or legs.

• Diaphragmatic hernia, an opening in the diaphragm that allows abdominal organs to move into the chest cavity. This may also cause lungs to be underdeveloped.

• Omphalocele, when the intestines or other abdominal organs protrude through the navel.

The study authors noted that the overall risk of having a child with a birth defect related to obesity is low. And mothers of babies born with gastroschisis (when organs protrude through a defect in the abdominal wall other than the navel) were less likely to be obese than mothers of babies without birth defects.

The findings are published in the August issue of Archives of Pediatrics & Adolescent Medicine.

It's not clear why the association between pre-pregnancy obesity and birth defects exists.

"We know that obese women have a higher risk of certain defects, but we don't know if obesity is the direct cause," Waller said. "There could be other explanations, such as different types of diet, different ways of dieting when they're dieting. We were not able to exclude women with diabetes, and that is a very strong risk factor for birth defects, so we think there may be undiagnosed cases of diabetes remaining with the study."

Future research will look at dieting techniques and the risk of birth defects, as well as any links between over-the-counter diuretics and appetite suppressants and birth defects.