The chances of a man having children dip past his 35th birthday, researchers have found.
The researchers, from the University of Washington in Seattle, found that damage to the genetic material containing sperm cells increases with age.
Unlike most other cells in the body, sperm cells are unable to repair this damage.
In addition, the researchers found that as a man gets older he loses his natural ability to weed out unhealthy sperm cells through a process known as apoptosis.
This means that there is a greater chance that a damaged sperm cell will successfully fertilise the female egg.
This could mean that the risk of miscarriage is increased or, at the other end of the scale, that children have a greater chance of developing mild abnormalities such as uneven teeth, or asymmetrical limbs.
Lead researcher Dr Narendra Singh told the BBC: “We found there is a significant change by the age of 35.”
Dr Singh’s team examined sperm quality in 60 men aged between 22 and 60. All had healthy sperm counts.
The researchers found that men aged 35 and older had higher concentrations of sperm with broken strands of DNA, and that the damage was greater.
In general, older men’s sperm was less active so they had less chance of fertilising an egg.
A recent study found that male infertility was just as likely as female infertility to prevent couples from having a family.
The proportion of British men aged over 40 becoming fathers increased by half in the 1990s. In 1999 one in 10 children was born to a father aged over 40.
It is known that a healthy lifestyle can have a positive effect on male fertility.
The findings are to be presented at a meeting of the American Society of Reproductive Medicine in Seattle.
Society president William Keye said: “While there is 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.”
Revision date: July 7, 2011
Last revised: by Jorge P. Ribeiro, MD