New research shows that women who exercise intensively during their first trimester of pregnancy are 3.7 times more likely to miscarry than pregnant women who don't break a sweat, according to a report from the French news agency, AFP.

Women who practiced high impact sports — such as jogging and racquetball — or exercised strenuously for at least seven hours a week ran the highest risk, according to the study from researchers at the University of Southern Denmark.

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The risk dropped off almost entirely after the 18th week of pregnancy. The study was based on interviews with 92,671 pregnant women in Denmark and was published in The International Journal of Obstetrics and Gynaecology. Swimming did not increase the risk of miscarriage, the study found.