"This study shows that men who wait until they are older to have children are not only risking difficulties conceiving, they could also be increasing the risk of having children with genetic problems," Andrew Wyrobek of Lawrence Livermore National Laboratory in California said in a statement.

For centuries, this tendency toward birth defects with aging was blamed on the woman, whose fertility plummets with age and disappears at menopause. Women are born with all their eggs, and these egg cells mature and ripen as women age.

Men Have Biological Clock

Older eggs are often defective and contribute both to lower female fertility and a tendency to genetic defects such as Down syndrome.

"Although it is well known that as women age, they are at increased risk for infertility, spontaneous abortion, and genetic and chromosomal defects among offspring, the association of male aging with these outcomes has been less well characterized," the researchers wrote in their report, published in the Proceedings of the National Academy of Sciences.

"Our research suggests that men, too, have a biological time clock only it is different," said Brenda Eskenazi of the University of California Berkeley's School of Public Health.

Abnormal DNA Fragmentation

For their study the researchers examined sperm samples from 97 men aged 22 to 80, all working at or retired from a government research laboratory.

They disqualified current cigarette smokers and men with current fertility or reproductive problems or who had undergone chemotherapy or radiation treatment for cancer.

They examined the sperm for obvious features which includes the ability to swim quickly, and harder-to-find qualities such as what is known as DNA fragmentation index a measure of damage that has been associated with male fertility, successful conception, and sustained pregnancy.

Men usually started to have an abnormal DNA fragmentation index at the age of 56, the researchers found.

They said it was important to understand the effects of men having children at ever-older ages.

"Since 1980 there has been about a 40 percent increase in 35- to 49-year-old men fathering children, and a 20 percent decrease in fathers under 30," they wrote.