According to a recent study carried out in Denmark women who exercise intensively during the first phase of pregnancy are 3.7 times more likely to miscarry.
The researchers say the women most at risk are those who do high impact exercise such as jogging and racketball but women who exercise strenuously for at least seven hours a week during the first trimester of pregnancy run the highest risk.
After the 18th week of pregnancy the risk almost completely disappeared.
The researchers say there was no increased danger of miscarriage to women who swam during any phase of pregnancy.
The study was based on interviews with 92,671 pregnant women in Denmark which were analysed by Anne Marie Nybo Andersen and colleagues from the University of Southern Denmark.
The researchers however do also say that expectant mothers should not be discouraged from taking mild to moderate exercise, but suggest their findings indicate the need for a review of exercise guidelines for pregnant women.
Some experts beg to differ and say the benefits of exercising during pregnancy are quite clear as it improves the cardiovascular system, and maintains muscle tone.
Current health guidelines in most countries including the United States, Britain and Denmark recommend physical exercise during pregnancy at a level similar to non-pregnant women.
The study is published in the International Journal of Obstetrics and Gynaecology.