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anencephaly
conotruncal heart defects
gastroschisis
hypothesis
limb reduction defects
neural tube defects
oral clefts

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Research has identified nutritional factors—particularly folic acid intake—as contributors to the complex origins of many birth defects. Starting with the hypothesis that overweight or underweight women may have unique nutrient profiles, we examined whether prepregnancy weight might be associated with birth defects.

Weight status is expressed as Body Mass Index (BMI), calculated as weight (in kilograms) divided by height (in meters) squared. About 10% of women are obese as defined by the Institute of Medicine's cutpoint for women: BMI > 29 kg/m2.

OBESITY RAISES RISK FOR NEURAL TUBE DEFECTS

We found obese women had nearly 2 times higher risk for neural tube defects—a finding since replicated by the Program and other researchers. The association was more pronounced for spina bifida (open spine) than for anencephaly (absence of the brain).

Nutritional and other factors didn't explain the study results:

bullet Folic acid intake: Women who were obese—whether or not they used vitamins—still had a greater risk than other women for neural tube defects.
bullet Weight reduction diets: Those who dieted in the months just before or after conception did not have a higher risk.
bullet Diabetes: Mothers with insulin-dependent diabetes have greater risk for neural tube defects, but excluding them from our analysis did not change results.

Extremely thin women did not have a greater risk for neural tube defects.

Will losing weight eliminate the extra risk seen among obese women? Not necessarily. There may be an underlying factor—nutritional, metabolic, or even genetic—linked to both obesity and neural tube defects; if so, weight loss alone might not correct this factor. We caution women to consult their physicians before attempting to lose weight—an unsupervised diet could actually increase risk by eliminating nutrients such as folic acid.

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OBESITY NOT A FACTOR IN OTHER BIRTH DEFECTS

We found no association between prepregnancy obesity and these conditions:

bullet Limb reduction defects
bullet Conotruncal heart defects
bullet Oral clefts (with or without additional birth defects).

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UNDERWEIGHT MOTHERS AT HIGHER RISK FOR GASTROSCHISIS

The younger the mother, the higher the risk for having a baby with gastroschisis, a serious abdominal wall defect. We theorized that teen mothers themselves may still be growing, thus competing with their pregnancy for nutrients.

bullet Babies with gastroschisis are born 3 times as often to mothers who are underweight (body mass index <18.1 kg/m2).
bullet Risk is reduced for overweight mothers.
bullet Other risk factors, including nutritional factors, did not explain these observations.

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