A new study has revealed that rates of pregnancy in women drop and risk of miscarriage increase if the father is over 40.

It has long been known that a woman's chance of reproducing declines with age once she is in her mid-thirties, but the new findings provide the strongest evidence to date that being an older father poses a risk as well.

Researchers in France monitored 21,239 cases of intrauterine insemination (IUI) -- a particularly effective type of artificial insemination -- in more than 12,000 couples.

As expected, they found that women over 35 showed significantly decreased pregnancy rates compared to younger women, as well as higher rates of miscarriage.

"But we also demonstrated that the age of the father was important in the rate of pregnancy, with a negative effect for men over 40," said Stephanie Belloc, a researcher at the Eylau Centre for Assisted Reproduction in Paris, and lead author of the study.

"And even more surprising, the proportion of miscarriages went up as well," she added.

Belloc was to present her research Monday at the annual meeting of the European Society of Human Reproduction and Embryology in Barcelona. They will be published in the British journal Reproductive Biomedicine.

In IUI treatment, sperm are separated from seminal fluid in a centrifuge. The "washed" sperm are then inserted directly into the uterus in order to enhance the chances of conception.