New findings released by separate American and French research teams indicate that declining quality of their sperm as men get older can affect their fertility. The U.S. 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. The researchers analyzed semen samples from 97 men from 22 to 80 years old. "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. He has determined that the DNA in sperm can fragment as men get older, increasing the risk of infertility and genetic abnormalities such as achondroplasia (dwarfism) or schizophrenia in offspring. The chance of miscarriage during the mother's pregnancy itself also increased. 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." 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." Researchers at INSERM, the French national health institute, also found that age affects fatherhood -- this time citing a specific threshold of concern. Male fertility plummets after 40, said Elise de la Rochebrochard, a public health epidemiologist who studied nearly 2,000 French couples undergoing fertility treatments. She found that the couples were 70 percent more likely to fail in their attempts at parenthood if the man was older than 40 -- regardless of the wife's age.