Hard training may reduce fertility in women, according to a new study by the Norwegian University of Science and Technology (NTNU).

The research by NTNU’s Human Movement Science Programme involved nearly 3,000 women. It found that overly frequent and hard physical exercise can reduce a young woman’s fertility.

However, it is thought the decrease in fertility is not permanent and only lasts as long as the hard training.

Sigridur Lara Gudmundsdottir, a PhD candidate at NTNU, 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.”

In women under 30, 24 per cent of those who trained to exhaustion had fertility problems and 11 per cent of women who trained almost every day reported the same.

Even when the data was adjusted for other possible factors, such as BMI, smoking age, marital status and previous pregnancies, the researchers found that women who trained every day had had a three and half times greater risk of impaired fertility than women who did not train at all.

However, the researchers pointed out that women who want to become pregnant shouldn’t give up all physical activity, but should be aware of the warning signs, such as a longer menstruation cycle or no menstruation at all.