Pre-pregnancy obesity has previously been linked to an increase in birth defects involving the brain and spinal cord. This association was seen in the new study, and researchers also reported an increase in heart, limb, and gastrointestinal birth defects among babies born to obese moms.
Obese women were at increased risk for delivering babies with seven of 16 major birth defects evaluated by the researchers.
But researcher D. Kim Waller, PhD, of the University of Texas School of Public Health, tells WebMD that the chance of delivering a child with a major birth defect is still low for obese moms.
According to Waller, based on the study’s findings, major birth defects could be expected in four out of 100 babies born to obese mothers. The average birth defect risk is closer to three in 100 births among babies born to normal-weight mothers, he notes.
"Obese women should not be overly alarmed by these findings, but it is important to understand the risk," she says. "While the absolute risk that an obese woman will have an infant with a birth defect is low, the contribution to the public health, given high rates of obesity in the U.S., is significant."
Twofold Rise in Spina Bifida
Interviews were conducted with 10,249 women in eight states who gave birth to babies with birth defects between 1997 and 2002 and with 4,065 women who delivered babies without birth defects during the same period.
The birth defect found to be most strongly linked to obesity in the study was the neural tube defect spina bifida.
Compared with babies born to normal-weight women, babies born to obese women in the study were twice as likely to have the neural tube defect even though obese moms were just as likely to take folic acid supplements prior to conceiving.
Taking folic acid before pregnancy dramatically reduces the risk of spina bifida and related neural tube birth defects.
A slightly lower increase in risk was identified for omphaleocele, a condition in which the intestines or another abdominal organ protrudes through the navel.
Obesity-related risk increases in the range of 20% to 50% were also seen for heart defects, limb abnormalities, malformations in the anal opening or urethra in boys, and a condition known as diaphragmatic hernia, which can interfere with lung development.
The study is published in the August issue of the Archives of Pediatric and Adolescent Medicine.
Diabetes and Birth Defects
Waller says undiagnosed diabetes could be largely responsible for the increase in birth defect risk seen among babies born to obese women in the study.
When the researchers reanalyzed the data excluding women who developed gestational diabetes during pregnancy, the maternal obesity-birth defect link was much smaller, she says, but it did not disappear entirely.
"Obese women need to follow the same recommendations as other women prior to becoming pregnant," she says. "But it would also be a good idea for them to see their doctor and get tested for diabetes. We know that many women have diabetes and don't know it. Identifying diabetes and controlling it prior to pregnancy can make a big difference."
March of Dimes acting director Michael Katz, MD, calls the study intriguing, but he adds that more research is needed to confirm the link between obesity and major birth defects.
"No matter what a woman’s weight, it is important to plan a pregnancy," he tells WebMD. "Planning ahead and taking steps to reduce modifiable risks can make all the difference."