The researchers looked at the soft drink habits of nearly 60,000 Danish women enrolled in a national study there from 1996 to 2002.
The investigators found a link between the intake of diet carbonated drinks and, to a lesser extent, diet noncarbonated drinks and delivering a baby early.
The study is published online and in the September print issue of the American Journal of Clinical Nutrition.
In the report, the researchers conclude: “Daily intake of artificially sweetened soft drinks may increase the risk of preterm delivery.”
The researchers defined preterm as delivering before 37 weeks’ gestation. They categorized the women into groups depending on beverage drinking habits: those who never drank soft drinks or those who drank less than one per week, one to six per week, one each day, two or three per day, or four or more daily.
In all, 4.6 percent of the women delivered early, and one-third of those deliveries were medically induced.
The team found no association between the premature delivery and the intake of carbonated drinks sweetened with sugar.
However, compared with those who never drank the beverages, women who downed four or more diet (artificially sweetened) carbonated drinks a day were 78 percent more likely to deliver early than women who never drank the beverages. And those who had four or more diet, noncarbonated drinks daily were 29 percent more likely to deliver early.
Those who had one or more carbonated diet drinks a day were 38 percent more likely to deliver early.
Why the diet drinks, especially, were linked with early delivery is not known, but the researchers speculate that the link may be driven by high blood pressure disorders in pregnancy. They note that other studies have found a link between soft drinks and high blood pressure in non-pregnant women.
The beverage industry took exception to the findings.
In a statement, Beth Hubrich of the Calorie Control Council said: “As a dietitian (and mom) I am deeply concerned that this information may unduly alarm pregnant women. The overwhelming majority of scientific literature shows that low-calorie sweeteners are safe for use in pregnancy.”
The study also doesn’t prove cause-and-effect, Maureen Story, senior vice president for science policy at the American Beverage Association, said in a statement. ”The authors themselves acknowledge the fact that their findings cannot demonstrate cause-and-effect,” she said.
But other experts said pregnant women may want to take heed of the study results. In a statement, Shelley McGuire of the American Society of Nutrition, said the findings “may be really important in terms of preventing premature births, especially those that are medically induced by a woman’s health care provider.”
She suggests pregnant women focus on water, juices and milk.
And in a statement, Dr. Alan R. Fleischman, medical director of the March of Dimes, said that “pregnant women should eat smart and make sure that most of their food choices are healthy ones. Artificially sweetened drinks don’t make most lists of healthy foods. As the authors point out, additional research is needed to understand the impact of these beverages on pregnancy and fetal development. Until that is clear, it is prudent for pregnant women to drink these beverages in moderation. They also should discuss with their doctors their risk of preterm birth and the signs and symptoms of preterm labor. “