Just also found this article from September 2007 while researching miscarriage on the NHS.uk/news website (a very useful site for information on health issues inthe news)
Pregnant women who do strenuous exercise, such as jogging or playing racket sports and ball games, more than treble their risk of miscarriage, newspapers reported.
The newspapers said vigorous high-impact exercise within the first few months of pregnancy was associated with the highest risk of miscarriage. Exercise in the later stages of pregnancy did not affect the risk.
The reports are based on a study on more than 92,000 women in Denmark that looked for a relationship between exercise and miscarriage. The study performed different analyses on the results, which resulted in conflicting findings. As a result, the researchers are themselves cautious about suggesting a link between exercise and miscarriage.
The relationship between exercise and miscarriage has not been proven beyond reasonable doubt by this study.