PARIS (AFP) - Women who exercise intensively during the first phase of pregnancy are 3.7 times more likely to miscarry than pregnant women who don't break a sweat, according to a recent study carried out in Denmark. 

Women who practiced high impact sports -- such as jogging and racketball -- or exercised strenuously for at least seven hours a week during the first trimester of pregnancy ran the highest risk, according to the study, published in The International Journal of Obstetrics and Gynaecology.

The risk dropped off almost entirely after the 18th week of pregnancy.

Women who swam during any phase of pregnancy also showed no increased danger of miscarriage, noted the study, based on interviews with 92,671 pregnant women in Denmark analysed by Anne Marie Nybo Andersen and colleagues from the University of Southern Denmark.

Expectant mothers should not necessarily be discouraged from taking mild to moderate exercise, and the results must be interpreted with caution, says study, cited by the British magazine New Scientist.

But the findings do argue in favour of a review of exercise guidelines for pregnant women, the authors suggest.

Currently, health authorities in most countries -- including the United States, Britain and Denmark -- recommend physical exercise during pregnancy at a level similar to non-pregnant women.