"Previous studies have shown an association with other chronic metabolic problems," said study author Dr. Liwei Chen, an assistant professor of epidemiology at Louisiana State University Health Sciences Center, in New Orleans. "This is the first to show an increased risk among pregnant women."

Gestational diabetes, known as glucose intolerance during pregnancy, is one of the most common complications of pregnancy. It increases the chances of lifelong diabetes for the woman and also can have permanent effects on the unborn child, Chen said. The report appears in the December issue of Diabetes Care.

"Other studies suggest that babies born to women who are diabetic during pregnancy have higher weight at birth and also higher rates of obesity and diabetes early in life," she added.

Chen, working with researchers at the Harvard School of Public Health and the U.S. National Institute of Child Health and Human Development, studied 10 years of medical records on a group of 13,475 women from the Nurses' Health Study II. After adjusting for known risk factors for gestational diabetes, such as age, family history and smoking, the researchers found that women who had more than five servings per week of sugar-sweetened cola beverages had a 22 percent higher risk of gestational diabetes than women who had less than one serving per month.

No such association was found for consumption of other sugar-sweetened beverages or artificially sweetened drinks.

It's not clear why only cola drinks are associated with the increased risk, Chen said. One explanation could be "the tremendous popularity of cola in the United States," she said.

The results are not conclusive, and "we need other studies to confirm our findings," Chen said. "We plan to study other populations, and we hope that other investigators start such studies."

But meanwhile, "I suggest that women reduce their intake of sugary beverages," she said. "Women need to be aware of the possible risk not only for their pregnancy but also of the long-term consequences for their babies."

Maureen Storey, senior vice president of science policy for the American Beverage Association, which represents soft drink companies, said she was skeptical about the findings.

"As with all epidemiological studies, the data do not show a cause-and-effect relationship," Storey said in a statement.

Sugar-sweetened beverage consumption has not been among the risk factors for gestational diabetes identified by "leading scientific bodies," Storey added.

"The key to a healthy pregnancy is seeking good medical care and having a healthy lifestyle before and during pregnancy," the statement said. "This includes eating a variety of foods and beverages in moderation along with getting regular physical activity."

According to the journal report, there are several potential explanations for the association. For example, sugar-rich foods or beverages can overload the body with glucose, which can impair the function of the beta cells of the pancreas, which make insulin that metabolizes glucose.

Sugar-sweetened beverages are the leading source of added sugar in the American diet, Chen said. The U.S. National Health and Nutrition Examination Survey found that soft drink intake more than doubled between 1977 and 2001 among 19- to 39-year-old Americans, going from 4.1 percent to 9.8 percent, and that those in this age group had the highest rate of soft drink consumption.