The study of nearly 60,000 pregnant women in Denmark showed those who had at least one serving of diet soda a day were 38 percent more likely to deliver preterm.
Pregnant women who had at least four diet sodas a day were 80 percent more likely to deliver early.
But the American College of Obstetrics and Gynecologists says artificial sweeteners are OK in moderation.
The study was published in the American Journal of Clinical Nutrition.
