July 30, 2008 -- Women diagnosed with diabetes before their pregnancy have a higher risk of giving birth to a baby with a birth defect or multiple defects than a woman who does not have diabetes before giving birth, according to a new study.
"For single [birth] defects, the risk is three to four times greater, and about eightfold for multiple defects," says Adolfo Correa, MD, MPH, PhD, the study's lead author and an epidemiologist at the CDC's National Center on Birth Defects and Developmental Disabilities.
Women with pregnancy-induced diabetes, called gestational diabetes, were also more likely to have a child with a birth defect, but generally only if their weight before getting pregnant was in the overweight or obese range, with a body mass index (BMI) of 25 or higher.
While the diabetes-birth defects link has been long known, this new research breaks new ground, says Correa.
"This is the largest study of its kind done on diabetes and birth defects," he tells WebMD. The scope was broader than other studies, too, since the researchers looked at those with diabetes as well as gestational diabetes. And they looked at nearly 40 defects, both cardiac and noncardiac.
Even so, another expert cautions that women, even those with diabetes and planning a pregnancy, should not become frightened by the latest research. Ninety-three percent of the birth defects weren't associated with maternal diabetes. Two percent of the children with single birth defects were born to mothers who had diabetes before their pregnancy, while 5% of those born with multiple defects were born to diabetic moms.
Diabetes and Birth Defects
Correa and his colleagues evaluated mothers of more than 13,000 infants with birth defects and nearly 5,000 infants without birth defects. The children were born between 1997 and 2003 and were participants in the National Birth Defects Prevention Study, which drew data from 10 birth-defect surveillance systems in 10 states.
They looked at whether a mother had diabetes, either type 1 or type 2, before becoming pregnant or whether she developed it during pregnancy. Women also reported other information, including height and weight, so their BMI could be calculated. Twenty-four mothers of the nearly 5,000 infants without birth defects had diabetes before pregnancy; 283 moms of the babies with birth defects had diabetes before pregnancy.
While diabetes contracted before pregnancy was associated with a wide range of birth defects, diabetes that came on during pregnancy was associated with a limited group of birth defects, Correa's team found.
In general, he says, women who got gestational diabetes tended to have children with birth defects only if their pre-pregnancy BMI had been 25 or higher.
Diabetes diagnosed before pregnancy was linked with about 50% of the birth defect categories analyzed.
Role of High Blood Sugar
Exactly why pre-pregnancy diabetes boosted birth defects risk so much isn't known. But experts say that high blood sugar (hyperglycemia) plays a role.
For instance, in animal studies cited by the researchers, a mother's high glucose (blood sugar) has been found to lead to the same in the embryo, causing biochemical abnormalities that increase oxidative stress and could lead to incomplete closing of the neural tube, in turn causing such neural tube defects as spina bifida.
"The new research confirms some early studies," says Janis Biermann, a spokeswoman for the March of Dimes who reviewed the study for WebMD. But the research also goes beyond that earlier research, she says, by studying a much larger group and by going into more detail about a variety of birth defects.
Women should take the new research as a call to take better care of themselves, Biermann says, to do what they can to reduce the risks.
Women shouldn't think birth defects are inevitable if they have diabetes before getting pregnant, she says. "Just because there is an increased risk of a baby having a birth defect if a woman has preconception diabetes doesn't mean it is going to happen. It just means there is a greater chance than if a woman doesn't have it."
Women who are already diagnosed with diabetes who hope to get pregnant can take crucial steps to beat the odds, she says. "It's important to take care of yourself, exercise, be at an optimal weight, plan your pregnancy, and make sure the diabetes is well controlled before you get pregnant."
Once you are pregnant, she tells women with diabetes, follow the same healthy habits and go for regular prenatal care. Those with diabetes, she says, also need to keep their regular appointments with their diabetes specialist.
About 1.85 million U.S. women of childbearing age have diabetes, the March of Dimes estimates.
The study is published in the American Journal of Obstetrics & Gynecology.