To investigate, researchers looked at nearly 60,000 Danish women who reported on their diet, including how many soft drinks they had each day, at around 25 weeks of pregnancy.
Around 5 percent of women delivered their babies before 37 weeks.
Women who had at least one serving of artificially sweetened soda a day while they were pregnant were 38 percent more likely to deliver preterm than women who drank no diet soda at all.
Women who had at least four diet sodas a day were nearly 80 percent more likely to deliver preterm. The association was the same for normal-weight and overweight women.
Because only diet soda was linked to preterm delivery, not sugar-sweetened soda, the findings suggest that the artificial sweetener itself, not soda drinking, could account for the relationship, the researchers say. However, they add, other possible causes for the link can't be ruled out.
While "diet" drinks are widely promoted as a healthy alternative to sugary sodas and juices, researchers note that there's been little research on the safety of regular consumption of artificial sweeteners in humans.
Soft drinks, both artificially sweetened and sugar sweetened, have recently been linked to high blood pressure, the researchers add, which increases the risk of premature delivery.
These latest findings were reported in the American Journal of Clinical Nutrition.
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