By Caroline Parkinson
Health reporter, BBC News, Barcelona
Previous studies have linked ageing to a lower chance of fatherhood
Scientists say they have found more evidence that men as well as women have biological clocks and that they start to tick in their mid-30s.
A French study of over 12,200 couples having fertility treatment suggests the chance of a successful pregnancy falls when the man is aged over 35.
It adds that the chance is significantly lower if he is over 40.
Previous studies have shown that both natural and assisted conception is more difficult if the man is over 40.
The researchers told a European reproductive health conference that it was likely the problems were caused by DNA damage in sperm.
The researchers studied couples who had sought treatment for infertility at the Eylau Centre for Assisted Reproduction in Paris between January 2002 and December 2006.
All were given intrauterine inseminations (IUI), also known as artificial insemination, where sperm is inserted into the womb when the woman is ovulating.
It is given to couples where the woman has no fertility problems and is less invasive than IVF.
The men's sperm were examined for quantity, their ability to move and swim and their size and shape.
Rates of pregnancy, miscarriage and births were recorded.
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In addition, the researchers analysed detailed data on the pregnancies, which allowed them to pinpoint factors associated with the man and the woman.
As expected, maternal age had an effect in women over 35, who had a significantly higher chance of miscarriage and lower rate of pregnancy.
But the team also found that, where the father was in his late 30s, miscarriages were more common than if the man was younger.
And if a man was over 40, the chances of a successful pregnancy were even lower.
For those couples, a third of pregnancies ended in miscarriage and only 10% of treatments resulted in pregnancies.
Dr Stephanie Belloc, who presented the work to the European Society of Human Reproduction and Embryology (ESHRE) conference in Barcelona, said: "This research has important implications for couples wanting to start a family."
She said such couples should be offered IVF (where an egg is fertilised in a lab dish), and where the outer membrane of the egg seems to block sperm with DNA damage, and ICSI (where a sperm is injected directly into an egg), where 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 also reduce the risk of miscarriage," she added.
Dr Allan Pacey, a fertility expert at Sheffield University and secretary of the British Fertility Society, said: "There is growing evidence from a number of studies to show that men are not totally immune from reproductive ageing.
"Previous studies of couples trying to conceive naturally or undergoing IVF have shown that men over the age of about 40 are less fertile than younger men. Moreover, if they do achieve a pregnancy their partners are more likely to miscarry.
"This study reinforces the message that men aren't excused from reproductive ageing."