Fertility starts declining around the age of 40 and its downward path is swifter among those who drink, smoke, take drugs and are overweight. This is according to a research report released in Australia by the fertility society of Australia

The chances are, you knew that - and it's also likely that you were thinking of women when you read the first sentence.

A recent study found only 2 per cent of Australians thought a man's age mattered in whether he could father children. But age and lifestyle factors affect the quality of sperm and the chances of fatherhood.

The Fertility Society of Australia's Anne Clark said sperm count had been the standard test of fertility for men but that doctors were coming round to viewing DNA breakages in sperm as a significant factor.

If more than 20 per cent of sperm is “fragmented” the chances of a partner becoming pregnant is significantly reduced. The usual lifestyle factors like drinking, smoking and being overweight were linked to DNA damage.

“This is one of the main reasons that when a man hits 40 the chance of his partner in her 30s conceiving either naturally or with IVF is halved,” Clark said. “But there's extremely little awareness of this, and it's hard to convince men this is an important part of the assessment process.”

She said that a change in lifestyle could boost the quality of sperm and aid procreation. But not delaying having children was always the best bet.

“The bottom line in all of this is that couples should consider conceiving earlier when their fertility is at optimum levels,” she said.