A Swedish study has found that pregnant women who drink five cups of coffee a day more than double their risk of a miscarriage during their first three months of pregnancy.

The data applies only to non-smokers. Researchers say the risk among smokers is already very high anyways.

The study was undertaken by the Karolinska Institute of Stockholm. Researchers interviewed 562 women who had miscarried and 953 who did not.

Dr. Sven Cnattingius lead the team. Cnattingius says his findings are only tentative but warns "reducing caffeine intake during early pregnancy may be prudent."

His study discovered women who drank one to three cups of coffee a day were 30 per cent more likely to miscarry. And for women who drank five cups or more, the risk climbed by 220 per cent.

Caffeine is passed to the fetus and in pregnant women, the substance stays longer in the body.

The study is published in this week's New England Journal of Medicine.