Male fertility starts falling after 35, say scientists who have found that pregnancy rates decrease and miscarriages increase when the father is over 35.

The French researchers followed over 20 000 intrauterine inseminations (IUIs), where the man’s sperm was inserted into the uterus to help couples get pregnant. In most cases the couples were being treated because of the husband's infertility.

The sperm of each partner was examined at the time of the IUI for a number of characteristics, including sperm count. Pregnancy, miscarriage and delivery rates were also carefully recorded.

They found, as expected, that the mother’s age was closely associated with a decreased pregnancy rate of 8.9% in women over 35 years, compared to 14.5% in younger women. Miscarriage rates were also typically affected by maternal age.

"But we also found that that the age of the father was important in pregnancy rates – men over 35 had a negative effect," says Dr. Stéphanie Belloc of the Eylau Centre for Assisted Reproduction, Paris. "And, perhaps more surprisingly, miscarriage rates increased where the father was over 35."

The effect of maternal age on the ability to conceive and on miscarriage rates is well known, but there is still controversy about the role of the father. Although there are many reports that show an overall decline in sperm counts and quality from decade to decade, up to now there has been no clinical proof that simply being an older man has a direct effect on a couple's fertility.

"We already believed that couples where the man was older took longer to conceive," says Dr. Belloc, "but a number of reasons had been put forward for this. Neither was there any definite evidence that miscarriage rates increased when the man was older.”

Belloc said she believes fertility treatment like IVF and ICSI should be suggested to infertile patients where either party is over 35 years of age.

“In IVF, the outer membrane of the egg seems to be an efficient barrier in preventing the penetration of sperm with DNA damage, and in ICSI, the best sperm can be selected out for use,” she explained.

“These methods, although not in themselves a guarantee of success, may help couples where the man is older to achieve a pregnancy more quickly, and also reduce the risk of miscarriage."