While some research has suggested that obese women have an increased risk of having a baby with a birth defect, a new study shows that diabetes may at least partly account for the link.

Studies on whether obesity raises the odds of birth anomalies such as spina bifida, cleft palate and heart defects have so far come to conflicting conclusions. One question is whether obesity, per se, is the problem - or whether certain factors associated with obesity are at work.

Type 2 diabetes, which is closely related to obesity, has been linked to a heightened risk of birth defects in a number of studies.

The new study, of nearly 42,000 women who gave birth between 1991 and 2004, found no association between mothers’ obesity and the risk of any major birth defect. However, there was a link seen with diabetes.

Women who’d had diabetes before becoming pregnant showed a nearly four-fold higher risk of having a baby with a birth defect than women without the disorder.

The findings, published in the journal Obstetrics & Gynecology, do not mean that women with diabetes generally have a high risk of having a baby with a birth defect.

The vast majority of babies in the study were born with no congenital defects; across the study period, the rate of any major anomaly was less than 1 percent among all women.

What’s more, past research has shown that well-controlled diabetes carries a lesser risk.

“We have known for some time that women who have pre-gestational diabetes have a reduction in the rate of congenital anomalies with improvement in (blood sugar) control in the pre-conception period and during pregnancy,” Dr. Joseph R. Biggio, Jr., the lead researcher on the current study, told Reuters Health in an email.

A 2007 study, for example, estimated the absolute risk of birth defects according to women’s A1C levels around the time of conception; A1C is a measure of blood sugar control over several months.

It found that women with A1C levels lower than 7 percent - which is generally recommended for people with diabetes - had a 2 percent to 3 percent chance of having a baby with a birth defect. That risk was 6 percent among women with A1C levels of 9 percent, and the two continued to climb in tandem.

Based on that evidence, diabetic women who are thinking about pregnancy should try to optimize their blood sugar control, said Biggio, director of the division of maternal fetal medicine at the University of Alabama at Birmingham.

For their study, Biggio and his colleagues analyzed data on 41,902 women who gave birth at their center between 1991 and 2004; the women were largely from the inner-city and the majority were African American. When the researchers separated the data into three five-year periods, they found that maternal obesity, diabetes and birth defects all increased over time.

Between 1991 and 1994, about 0.4 percent of babies were born with a major congenital anomaly, such as a defect of the heart, spine, brain, lungs or digestive system. That rate was just over 0.8 percent between 2000 and 2004.

At the same time, the prevalence of obesity increased from 29 percent to 41 percent, while pre-pregnancy diabetes rose from just over 1 percent of all women to just over 3 percent.

Of women with obesity and diabetes in the 2000-2004 period, diabetes appeared to account for about three-quarters of the birth defect risk.

There are several theories on why diabetes is related to birth defects, Biggio said. Excess blood sugar, he explained, is delivered to the embryo early in pregnancy, and that may end up spurring an overproduction of cell-damaging substances called free radicals. The extra sugar may also result in metabolic byproducts that interfere with signaling mechanisms critical to embryonic development, Biggio noted.

SOURCE: Obstetrics & Gynecology, February 2010.