Spending too much time in the gym can reduce a woman's chances of having children, a study shows.
Researchers found that 'superwoman work-outs' made fertility problems three times more likely.
While experts agree that a certain amount of physical exercise has obvious health benefits, it is believed that too much saps the body of the energy it needs for a successful pregnancy.
The findings were made by Norwegian University of Science and Technology after a study of 3,000 women.
Although it is known that some elite female athletes have problems starting a family, other women who push themselves to the limit also appear to be affected.
In a survey, the women were questioned about the frequency, duration and intensity of their fitness regimes between 1984 and 1986.
In a follow-up ten years later, they were asked about their pregnancies.
Sigridur Lara Gudmundsdottir, who led the study, said: 'Among all these women, we found two groups who experienced an increased risk of infertility.
There were those who trained almost every day, and there were those who trained until they were completely exhausted.'
Those who did both had the highest risk of infertility.
Even after taking other factors such as age, weight, marital status and smoking into account, figures showed those who trained the hardest were three times more likely to have fertility problems than those who exercised moderately.
Younger women appeared to be more vulnerable to the risk. Among the under-30s who exercised the most, a quarter were unable to conceive during their first year of trying, compared to the national average of roughly seven per cent.
However, the negative effects of a punishing routine did not appear to be permanent.
'The vast majority of women in the study had children in the end,' Gudmundsdottir said.
'And those who trained the hardest in the middle of the 1980s were actually among those who had the most children in the 1990s,' she added.
It was not known whether this was because the women had simply changed their activity levels or because their hormone profile improved with time.
There was no evidence of impaired fertility through moderate amounts of exercise.Gudmundsdottir advised that women who want babies should still maintain their fitness - but 'ease off a bit' and avoid extremes.
'We believe it is likely that physical activity at a very high or very low level has a negative effect on fertility, while moderate activity is beneficial,' she said.