Spending too much time in the gym could make pregnancy harder for women trying to conceive.
A study of 3,000 women suggests extreme workouts could increase the risks of fertility problems three fold.
Of the most active women in the study under 30, a quarter could not conceive during their first year of trying, compared with the national average of 7%.
The women were asked about the frequency, intensity and duration of their fitness regimes between 1984 and 1986. In a follow-up 10 years later, they were asked about their pregnancies.
“Among these women, we found two groups who experienced an increased risk of infertility,” said head researcher Sigridur Lara Gudmundsdottir. “There were those who trained almost every day and there were those who trained until they were exhausted. Those who did both had the highest risk of infertility.”
Even after age, weight, marital status and smoking were taken into account, the hardest trainers were three times more likely to have fertility problems than moderate exercisers.
However, the affects were not permanent and the majority of women went on to have children eventually.