Men who want to become fathers should not delay having children too far beyond their mid-30s, an Australian fertility expert says.
Male fertility is one of the topics being discussed at a Fertility Society of Australia conference in Auckland this week.
A professor at the University of Newcastle in New South Wales says men - like women - should have children early in life, if possible.
John Aitken says the early 20s is ideal biologically, and after the age of 35 men have a significantly increased risk of DNA damage to their sperm which can make them infertile.
"They don't stop producing sperm, but the quality of those gametes deteriorates with age and the likelihood that the offspring will have some kind of genetic disease increases with the age of the father."
Professor Aitken says men with damaged sperm can pass on conditions to their children, including schizophrenia, bipolar disorder, epilepsy and autism.
It is rare for this to happen, but 1 in 20 men in modern societies experience some form of infertility, he says.