Older fathers warned

The biological clock is ticking for men as well as women. Researchers have produced evidence to show that sperm are more likely to be genetically damaged in older men.

Tests involving 66 men aged between 20 and 57 showed that the older men had more sperm with highly damaged DNA than the younger ones, according to a report in the medical journal Fertility and Sterility.

The older men's sperm were also less likely to undergo a self-destruct mechanism called apoptosis, a natural protective process designed to

get rid of damaged cells. It may not be chronological age alone that affects sperm quality, but the environmental damage that comes with age, said the research team at the University of Washington, Seattle.

French research has previously suggested that men over 40 are three times more likely to father a baby with Down's syndrome than younger men.

And studies have shown that men's fertility declines with age, in much the same way as women have less chance of conceiving as they get older.