The French study of over 12,200 couples having fertility treatment suggests the chance of a successful pregnancy falls when the man is aged over 35.

The quality of sperm begins to deteriorate in the mid-thirties and by the time a man is 45, one in three pregnancies ends in miscarriage regardless of the age of the mother, the study says.

The researchers studied couples who had sought treatment for infertility at the Eylau Centre for Assisted Reproduction in Paris between January 2002 and December 2006.

All were given intrauterine inseminations (IUI), also known as artificial insemination, where sperm is inserted into the womb when the woman is ovulating.

It is given to couples where the woman has no fertility problems and is less invasive than IVF.

The men’s sperm were examined for quantity, their ability to move and swim and their size and shape. Rates of pregnancy, miscarriage and births were recorded.

In addition, the researchers analysed detailed data on the pregnancies, which allowed them to pinpoint factors associated with the man and the woman.

As expected, maternal age had an effect in women over 35, who had a significantly higher chance of miscarriage and lower rate of pregnancy.

But the team also found that, where the father was in his late 30s, miscarriages were more common than if the man was younger. And if a man was over 40, the chances of a successful pregnancy were even lower.

The report has left experts arguing about how far modern lifestyles are to blame for failing sperm.

The study says it is not just the ability to have children that may or may not be affected by environmental factors.

Stephanie Belloc, lead author of the study, said: “Until now, gynaecologists only focus on maternal age, and the message was to get pregnant before the age of 35 or 38 because afterwards it would be difficult. But now the gynaecologists must also focus on paternal age and give this information to the couple.”

According to the study, men’s sexual peak is at 22 at least in terms of testosterone.

After that, levels of the male hormone fall by around one per cent a year, with the amount of bio-available testosterone halving between the ages of 25 and 75, according to the study published in The Independent.

“But peak hormones don’t necessarily mean peak sexual performance,” says the psychotherapist Michael Perring.

Peak sexual performance, he says, is obviously closely associated with environmental factors, including availability of partners.