It has long been known that a woman's chance of reproducing declines once she hits 35, but now scientists have found that men who have some forms of fertility treatment in their 30s suffer the same fate.

A study by Laboratoire d'Eylau, a centre for assisted reproduction in Paris, followed more than 21,000 men who had intrauterine inseminations at fertility clinics. It found the process, where semen is washed to extract the sperm, resulted in a decrease in pregnancies and an increase in miscarriages.

All the men in the study, which will be presented today at the annual conference of the European Society for Human Reproduction and Embryology, were aged over 35.

"We already believed that couples where the man was older took longer to conceive," said the study's author, Stephanie Belloc.

"But how DNA damage in older men translates into clinical practice has not been shown up to now. Our research shows for the first time that there is a strong paternal age-related affect on [intrauterine insemination] outcomes and this information should be considered by both doctors and patients in assisted reproduction."

She said sperm with DNA damage, common in older men, could still enter the egg during intrauterine insemination, which could result in a failure to conceive or a miscarriage.

But during in vitro fertilisation, the zona pellucida, or outer membrane of the egg, was an efficient barrier in preventing the penetration of sperm with DNA damage.

"And in ICSI (intracytoplasmic sperm injection), the best sperm can be selected for use. 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 reduce the risk of miscarriage," Dr Belloc said.

She followed 21,239 patients, and examined the sperm of each partner for count, motility and morphology. Pregnancy rates, miscarriage and delivery rates were also recorded.

"Some recent studies have established a relationship between the results of [intrauterine insemination] and DNA damage, which also correlated to a man's age, suggesting it might be an important factor, but until now there was no clinical proof. We have now found that the age of the father was important in pregnancy - men over 35 had a negative effect."