Mother's Obesity

Women with a BMI of 25 — 145 pounds for that 5-foot-4 woman — up to 29.9 are considered overweight, but the new analysis did not link that weight range and a higher risk of birth defects. "That's not necessarily because overweight doesn't have a risk attached to it," but studies to answer that question haven't been done, says co-author Judith Rankin, an epidemiologist at Newcastle University in the United Kingdom. Rankin and her co-authors came up with possible explanations for the link between obesity at conception and a higher risk of birth defects:

•Obesity is a strong risk factor for type 2 diabetes, and diabetes in pregnant women is an established risk factor for birth defects, especially of the central nervous system and the heart.

•Performing ultrasounds of obese pregnant women is more difficult, so perhaps they might not terminate pregnancies because of fetal defects as often as thinner women.

•Research has found an association between maternal obesity and nutritional deficiencies, specifically reduced foliate levels. Women of childbearing age are advised to take 400 micrograms of folic acid a day to protect against spina bifida, but maybe that's not enough if they're obese, Rankin says.

But James Mills, senior investigator in the epidemiology branch at the Eunice Kennedy Shriver National Institute of Child Health and Human Development, says there's no evidence that bigger doses of folic acid for obese women would help. Back in 1994, Mills co-wrote the first study to link obesity in pregnancy to birth defects. It found that obese women were 2½ times more likely to have a baby with spina bifida than normal-weight women, fairly consistent with Rankin's finding.