It isn't only women who face a ticking biological clock when planning parenthood.

New research has found that as men age, the quality of their sperm deteriorates, making it more likely they will have trouble becoming fathers and increasing the possibility of having a child with dwarfism.

The study, led by Andrew Wyrobek of Lawrence Livermore National Laboratory and Brenda Eskenazi of the University of California, Berkeley, School of Public Health, appears in this week's online edition of the Proceedings of the National Academy of Sciences.

Women's biological time clock has long been known, with older women having an increased risk of miscarriage and of producing children with genetic defects such as Down Syndrome.

"Our research suggests that men, too, have a biological time clock — only it is different," Eskenazi said in a statement. "Men seem to have a gradual rather than an abrupt change in fertility and in the potential ability to produce viable, healthy offspring."

Both men and women have been postponing parenthood in recent years. Since 1980, the researchers said, birth rates have increased 40 percent for men aged 35 to 49, while there has been a decline in births involving men under 30.

The same team had previously found that as men age their sperm count declines and their sperm becomes less active.

The new report looked at 97 men aged 22 to 80 and found increased fragmentation of the DNA in sperm as men age.

"This study shows that men who wait until they're older to have children are not only risking difficulties conceiving, they could also be increasing the risk of having children with genetic problems," said Wyrobek.

Unlike older women, the changes in sperm did not increase the chance of producing a child with Down Syndrome, they found. But some older fathers did have an increased risk of having children with dwarfism and "a small fraction of men are at increased risks for transmitting multiple genetic and chromosomal defects."

The study was primarily funded by several grants from the National Institute of Environmental Health Sciences, part of the National Institutes of Health, and the U.S. Environmental Protection Agency.

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