MEN who put their career before having a family should beware: the ticking of the biological clock is as important for fertility in men as it is in women.

American scientists have discovered that genetic damage to sperm routinely starts to cause infertility in men as young as 35. The strongest biological evidence yet for a significant drop in male fertility in the late thirties is a warning to the increasing number of grey-haired fathers who are leaving it later to have children.

The popular worry that career women risk losing the chance to have children has long been supported by infertility research focusing on how the quality of women’s eggs deteriorates with age. Researchers from the University of Washington in Seattle have now provided the first firm molecular explanation for why childless career men should worry too. The chances of having a baby are reduced if the man is in his late thirties or forties.

The study, led by Narendra Singh and unveiled at the American Society for Reproductive Medicine conference in Seattle today, examined the sperm of 60 volunteers aged between 22 and 60. All the men had healthy sperm counts.

Dr Singh’s team found that, whatever the sperm count, its genetic quality was closely related to age, with a cut-off point for serious damage of about 35.

Men in the older group had higher concentrations of sperm with broken strands of DNA, more acute levels of such genetic damage and their immune systems were much less efficient at weeding out faulty sperm by programmed cell suicide, or apoptosis. The sperm of the older men were also less vigorous swimmers.

Clare Brown, of the British infertility charity Child, said the findings cast new light on the often overlooked problem of male infertility.

“About a third of all infertility is male factor,” she said. “Male-factor infertility is more prevalent than people think. It’s not generally in the public’s mind that male sperm quality does indeed go down with age, from, as we now see, the age of 35.”

William Keye, president of the American Society for Reproductive Medicine, said that men concerned about their fertility should avoid activities such as smoking that may damage the DNA of their sperm. He added: “While there’s nothing anyone can do about getting older, men who want to retain their own best capacity to father children should try to minimise contact with toxic agents and maintain a healthy lifestyle.”

The proportion of British men aged over 40 becoming fathers increased by half in the 1990s. In 1999 one in ten children was born to a father aged over 40. The number of children born to fathers over 40 has risen by nearly a third to 42,000 a year in the past 20 years. Older fathers include David Jason, who had his first child at 61, Tony Blair, John Humphrys, David Bowie and Mick Jagger. James Doohan — Scotty from Star Trek — was an 80-year-old great-grandfather when his wife gave birth to his seventh child.

The findings do not suggest that most men who wait until after 35 to try for children will have problems, particularly if the man’s partner is in her twenties or early thirties. But the study does alert fertility doctors to another potential problem when older couples have difficulty in conceiving.

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