While exercise during pregnancy is advised, a Danish study shows that intense effort during the early stages could lead to miscarriage.

Researchers at the University of Southern Denmark in Odense interviewed more than 90,000 pregnant women on their exercise habits: frequency, length and type of physical effort, as well as on the outcome of their pregnancy.

They found that the women who practiced high-impact sports such as ball games, jogging and racket sports were at greater risk of suffering a miscarriage, as were those who did more than seven hours of exercise a week.

These women were approximately three-and-a-half times more likely to lose their baby, compared to those who did not exercise at all.

The researchers write in the study, published in the New Scientist magazine, that swimming did not increase the risk at all. No link was found between exercise and miscarriage after 18 weeks.

The study was originally published in the British Journal of Obstetrics and Gynaecology, and the authors wrote: “The results of this study suggest that leisure time exercise during pregnancy, and particularly high-impact exercise, is associated with an increased risk of miscarriage in the early stage of pregnancy,” reports BBC News.

The researchers also allowed that several aspects had not been taken into consideration in their study. They said the format of the research, simply asking women to remember the exercise they did during pregnancy, was imperfect.

Likewise, they did not take into consideration the fact that women who later miscarry usually do not experience morning sickness, which could explain why they were able to exercise during the early stages of pregnancy. This would mean that it was not the physical effort that led to a miscarriage.

Health authorities in most countries advise pregnant women to exercise moderately and only for as long as they are comfortable. Sports that involve the risk of being hit in the abdomen should be avoided.

The authors emphasized that women should not restrain from mild to moderate exercise.