Strenuous exercise early in pregnancy could triple the risk of miscarriage, according to Danish researchers.
Government advice is to stay active in pregnancy
The research found jogging, ball games and racket sports all increased the risk - as did more than seven hours of exercise a week.
However, current government advice is to continue exercise, where possible, throughout pregnancy.
And one UK expert took issue with the research, highlighted in New Scientist magazine, arguing running was safe.
The study was carried out at the University of Southern Denmark, who quizzed more than 90,000 women on their exercise regime, then linked the answers to the outcome of their pregnancies.
Compared with those who did no exercise at all, women who played "high-impact" sports or who exercised for more than seven hours a week were approximately three-and-a-half times more likely to miscarry.
Miscarriage is far rarer later in pregnancy, and after the 18-week mark, the link between exercise and miscarriage disappeared.
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."
They did concede, however, that the format of the research - asking women to recall what exercise they did during pregnancy - was imperfect.
It also did not allow for the fact that women who later miscarry are far less likely to experience morning sickness, which might be a reason they were able to exercise early in pregnancy.
Some forms of exercise did not appear to increase the risk - including swimming, one of the most popular among pregnant women.
The government's official advice is for women to keep up their normal daily physical exercise routine for as long as they feel comfortable.
It suggests that women who were inactive before pregnancy shouldn't embark on a new strenuous fitness regime, but should aim for a little moderate exercise every day.
Alison Merry, a former midwife, runs a firm, Blooming Fit, which designs exercise programmes for pregnant women.
She said that while she would not normally recommend any sport which involved the risk of an impact to the abdomen of a pregnant woman, she was comfortable with the idea of pregnant women continuing to jog or run during early pregnancy.
She said: "The benefits of exercising during pregnancy are clear - it improves the cardiovascular system, and maintains muscle tone.
"While I would say that getting a hockey ball in the stomach is not a good idea, I can't think of any reason why jogging would be harmful."