Couples trying to have a baby when the man is over 40 will have more difficulty conceiving than if he is younger, French researchers said on Sunday.

Doctors know a woman's age plays a key role but the findings presented at the European Society of Human Reproduction and Embryology conference suggest the paternal impact is stronger than earlier thought, Stephanie Belloc and colleagues said.

"Our data give evidence for the first time, for a strong paternal effect on IUI (intrauterine insemination) outcome either on pregnancy rates but also on miscarriage rates," Belloc and her team from the Eylau Centre for Assisted Reproduction in France said.

Other researchers have indicated that an overall decline in sperm counts and quality as a man ages is a factor but until now there has been little clinical proof that simply being an older man has such a big effect on fertility, the researchers said.

The French team analysed samples taken from more than 21,000 so-called intrauterine inseminations in which the sperm are washed or spun in a centrifuge to separate them from the seminal fluid and then inserted directly into the uterus.

The team examined the quality of the sperm and then tracked pregnancy, miscarriage and delivery rates. They found the paternal impact on miscarriage was much stronger when men passed age 40, said Yves Menezo, who worked on the study.

As expected, older women were less likely to get pregnant and had more miscarriages than younger ones, but surprisingly the risk of miscarriage was also far higher for couples in which the man was past 40, about 35 percent. The risk from a man younger than 30 is about 10 to 15 percent, Menezo added.

"We have known there was a paternal effect for a while but we didn't expect to find these kind of miscarriage rates," he said in a telephone interview.

The researchers do not know exactly why but said a link between a man's age and DNA decay in sperm that causes it to fragment could be a likely explanation.

The sperm they studied showed that many samples taken from men over 40 had defects that could cause miscarriage, the researchers added.

"Until now, gynaecologists only focused on maternal age, and the message was to get pregnant before the age of 35 or 38 because afterwards it would be difficult," Belloc added.

"But now the gynaecologists must also focus on paternal age and give this information to the couple."