A new study shows that a man's fertility starts to fall in his mid 30s, providing more evidence that like women, men, too, have a kind of biological clock that can play a big role in a women's chances of getting pregnant.
New research suggests that men may become less fertile as they reach their late 30s.
Researchers in France found that a male's fertility starts to decline as he enters his mid-30s and is significantly lower if he is over 40.
The study included more than 12,200 couples being treated for infertility at the Eylau Center for Assisted Reproduction in Paris. The women were given intrauterine inseminations, or IUIs, also known as artificial inseminations, where sperm is inserted into the uterus when the woman is ovulating. This type of treatment is typically given to couples if the woman has no fertility problems.
Scientists monitored more than 21,000 of these procedures between January 2002 and December 2006 and recorded rates of pregnancy, miscarriage and births.
If a woman was over 35, the rate of pregnancy was lower. This was expected: maternal age has been long been linked to a couple's rate of pregnancy and chances of miscarriage.
But the study also found that if the father was in his late 30s, the chances of a successful pregnancy went down. Ten percent of treatments led to pregnancy in fathers over 40.
Researchers said the problems were likely the result of DNA damage and fragmentation in sperm, which can lead to pregnancy failure and miscarriage.
Dr. Peter Schlegel, chairman of urology at the Weill Cornell Medical Center in New York, works at the Male Center for Reproductive Medicine. He told ABC News it is possible that there is a link between DNA damage and age.
"As men get older, there is an increase in the risk of having that fragmented DNA," Schlegel explained.
The findings were presented today at the European Society of Human Reproduction and Embryology conference in Barcelona, Spain, and represent the strongest evidence to date that age can affect male fertility.
Dr. Stephanie Belloc, who presented the study, said, "This research has important implications for couples wanting to start a family."
Belloc went on to report that although gynecologists have always placed emphasis on maternal age, now paternal age will increasingly become a factor in reproductive medicine.
"The message was to get pregnant before the age of 35 or 38, because afterward it would be difficult. But now the gynecologists must also focus on paternal age and give this information to the couple," she said in her presentation.
The U.S. Census Bureau, which compiles national health statistics, does not publish data on the age of first-time fathers, according to Andy Hait of the Census Bureau, but a handful of reproductive studies show that the average ages of men having children is going up.
Schlegel told ABC News it is becoming increasingly common for males to be treated for infertility.
"Couples are older when they try to have children than they were in the past, so it's a more common situation for us to encounter," he said. Schlegel also said there is more research to be done on how male age affects embryos and birth defects.
Schlegel went on to say the relationship between age and male fertility is not a new concept but has been masked by the focus on the connection between female fertility and age.
"The changes in male fertility rates, as they relate to age, are not as dramatic as in female fertility rates -- which are massive and marked," said Schlegel, "but there is a clear link between age and male fertility."