The results of a study involving more than 85,000 pregnant women surprised researchers as it was thought exercise would have a beneficial effect.

Pregnant women are recommended to take 30 minutes of moderate intensity exercise per day and the latest data suggest that exceeding that by even a small amount was linked to pre-eclampsia.

The condition affects one in 14 women to some extent but in around one in 100 pregnancies it develops into serious complications and the baby must be delivered as quickly as possible. Pre-eclampsia causes the deaths of up to ten mothers a year in the UK and around 1,000 babies.

Danish and Norwegian researchers looked at medical data of thousands of women in Denmark who were pregnant between 1996 and 2002.

Jogging for more than one hour and 15 minutes a week more than doubled the risk of pre-eclampsia and this meant seven per cent of women who did this much exercise developed the condition.

Women with high levels of physical activity, 4.5 hours a week to seven hours a week, were 65 per cent more likely to have severe pre-eclampsia.

This means 18 per cent of women who do this much exercise will develop the condition.

Women who did more than seven hours a week of exercise were 78 per cent more likely to have the condition, meaning 29 per cent of women who do this much exercise will develop severe pre-eclampsia.

Senior author, Dr Sjurdur Olsen, from the Statens Serum Institut in Copenhagen, Denmark and Adjunct Professor of Nutrition at the Harvard School of Public Health in Boston, USA said: "Recommendations have been issued in several countries that pregnant women should exercise at least 30 minutes each day. An important underlying contention is that this can reduce risk of pre-eclampsia. In our study, we were unable to substantiate that physical activity in early pregnancy has a protective effect against pre-eclampsia.

"Another unexpected finding was that leisure time exercise, in amounts that were only slightly higher than the recommended amount, seemed even to be associated with an increased risk of severe types of pre-eclampsia.

"Further research is needed in other large prospective cohort databases which are now emerging in several countries. Until that has happened, recommendations in the field should remain unchanged."

It is thought that excessive exercise in pregnancy may increase stress on the body leading to raised blood pressure.

The findings are published in the British Journal of Obstetrics and Gynaecology.

Professor Philip Steer, editor-in-chief of 'BJOG: An International Journal of Obstetrics and Gynaecology', said: "Clinical guidelines in the UK stress that selective and moderate exercise during pregnancy can be beneficial. These include aerobic and strength-conditioning exercises. While general fitness is a good thing in many respects, these data suggest that it may be unwise to exercise to peak fitness levels.

"This new research is useful as it provides us with an indication of how much exercise pregnant women should take. As with everything in life, too much of a good thing can be bad for you, and moderation in all things remains a good policy."