Research carried out on almost 60,000 pregnant women in Denmark found that those who drank artificially sweetened soft drinks, whether fizzy or still, were more likely to give birth early.
It was found that those who drank one serving per day of artificially sweetened fizzy drink were 38 per cent more likely to give birth before 37 weeks gestation and those who consumed four servings a day were 78 per cent more likely to have their baby prematurely.
The effect was weaker for still artificially soft drinks and there was no link between premature birth and sugar-sweetened drinks, they said.
It is thought the artificial sweeteners are broken down in the body into chemicals which may change the womb, the researchers said.
The study was published in the American Journal of Clinical Nutrition.
Dr Shelley McGuire, spokesman for the American Society of Nutrition, said: