Feb. 10, 2009 -- Women who are obese during pregnancy have a higher risk than normal-weight women of having babies with certain birth defects, including neural tube defects such as spina bifida, heart problems, and cleft palate and lip, according to a new review.
"It is important to note that birth defects are a rare event and occur in 2%-4% of pregnancies, so the risk remains very low," says Judith Rankin, PhD, a study co-author and a reader in material and perinatal epidemiology at the University of Newcastle in England. "The last thing we want to do is scare women.”
Rather, the goal is to inform them, she says, and to encourage women who are obese to get preconception counseling about weight loss.
The new report, published in this week's Journal of the American Medical Association, is a review of previously published work. Rankin and her colleagues culled the medical literature, pooled the results of 18 studies, and reviewed the findings of 39 other studies to determine if the association between obesity and birth defects still held up. It did.
Obesity & Birth Defects: The Study
Rankin's team undertook the study because of the growing problem of obesity in women of childbearing age. In the U.S., one-third of women 15 and older are obese, the authors note, and those numbers are expected to rise.
"This is a very important public health issue given the growing numbers of women who are obese at the start of pregnancy," says Rankin, who notes that obesity is also increasing in the U.K.
Obesity & Birth Defects: Results
- The risk of spina bifida was more than two times as high for obese pregnant women, and the overall risk of neural tube defects was nearly twice as high.
- The risk of cardiovascular defects was 30% higher.
- The risk of cleft lip and cleft palate, either singly or together, was about 20% higher.
- The risk of hydrocephaly (an abnormal buildup of fluid in the brain) was 60% higher.
- Limb reduction abnormalities were 30% higher.
The definitions of overweight and obese differed somewhat from study to study, but many studies used those set by the World Health Organization -- a body mass index or BMI of 25 and above for overweight and 30 and above for obese.
More research is needed to determine if the link between excess weight and birth defects holds for overweight women. "There isn't the same amount of research evidence for overweight as there is for obesity,'' Rankin says.
Obesity & Birth Defects: Explaining the Link
Exactly how obesity increases birth defect risk isn't known, but the researchers offer possible explanations.
- Because maternal diabetes is known to increase the risk of birth defects, and obese women are at higher risk for type 2 diabetes, the mother-to-be may have undiagnosed diabetes.
- Obese women have been shown to have nutritional deficiencies, especially reduced levels of folate, which is important to prevent neural tube defects. Obese women may need more than the amount routinely recommended to prevent birth defects.
Obesity & Birth Defects: Other Opinions
The new review further confirms what physicians have known for a long time, says Sina Haeri, MD, a clinical instructor of maternal-fetal medicine and a fellow at the University of North Carolina at Chapel Hill School of Medicine.
The problem with some of the previous studies, he tells WebMD, is that they had some methodological weaknesses. "So we took it all with a grain of salt," says Haeri, who recently reported that teen moms who are obese are more likely to be diagnosed with diabetes during the pregnancy (gestational diabetes) and to have cesarean deliveries.
In the new analysis, he says, the U.K. researchers took all the smaller studies and looked at them together and still found the obesity and birth defects link.
The new analysis confirms in a convincing way what physicians have been observing and studies have been suggesting for a few years, says Peter Bernstein, MD, a maternal-fetal medicine specialist at Montefiore Medical Center and Albert Einstein College of Medicine, New York.
Obesity & Birth Defects: Preconception Counseling
Women who are obese should have preconception counseling to minimize the risks of their excess weight to the newborns, Rankin says.
That is also the opinion of the American College of Obstetricians and Gynecologists. The organization's committee on obstetric practice issued its committee opinion on "Obesity in Pregnancy" in September 2005, recommending preconception counseling. Obese women should be informed of the risks associated with maternal obesity, be screened for gestational diabetes, and be assessed for the need for supplements of vitamins and minerals, including folate. Obese women should be advised to gain less weight than other women -- 15 pounds compared to 25 to 35 for women who were normal weight before pregnancy -- the opinion says.
"The most important prenatal visit is probably the one that happens before the woman gets pregnant," says Bernstein, who serves on the CDC's Select Panel on Preconception Care.
"It is not advisable to try to lose weight while pregnant," Rankin says. "The important thing is to have a sensible diet and try to eat healthfully."