Women who undertake strenuous exercise during the first few months of pregnancy could be more than trebling their risk of miscarrying, according to scientists.

   Interviews with more than 90,000 women in Denmark showed that those who exercised intensively were three-and-a-half times as likely to lose their babies as those who did no exercise.

   Activities such as jogging, racket sports or ball games carried the greatest risk of a miscarriage, the study found, as did exercising for more than seven hours a week.

   The researchers, led by Anne-Marie Nybo Andersen of the University of Southern Denmark in Odense, carried out the study, published in the British Journal of Gynaecology, because women are keen to know if they can continue exercising when pregnant.

   The scientists analysed interviews given by 92,671 pregnant Danish women over recent years in which they were asked what physical activities they engaged in and how long they spent exercising.

   Almost half of them exercised during pregnancy, mostly spending between 75 and 149 minutes a week on low-impact activities such as walking, aerobics, cycling, horseriding or swimming.

   Further analysis found the highest risk among those who exercised more than 7 hours a week and those who did "high-impact" activities such as running.

   However, swimming did not appear to increase the danger at all, nor did exercising after 18 weeks of pregnancy.

   The researchers concluded that the results did seem to show a clear link between exercise in early pregnancy and miscarriage: "The mode of action is unknown, but the fact that high-impact exercise seems to be associated with the highest risk of miscarriage indicate that jolts produced while exercising plays a role."

   However, they added: "It is too early to draw any public health inferences. Many positive effects of exercise are well-established and the findings of this study need to be replicated."

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