Dr. Stephanie Belloc of the Eylau Centre for Assisted Reproduction in Paris says this is the first time such a strong paternal effect on reproductive outcomes has been shown.

Belloc and colleagues tracked 21,239 intrauterine inseminations, or IUI. In IUI the sperm is "washed," or spun in a centrifuge, to separate them from seminal fluid, and then inserted directly into the uterus. If the sperm are not washed they can be expelled.

The sperm of each partner were examined at the time of the IUI for sperm count, motility and morphology.

Maternal age was closely associated with a decreased pregnancy rate of 8.9 percent in women over age 35, compared to 14.5 percent in younger women.

"But we also found that that the age of the father was important in pregnancy rates -- men over 35 had a negative effect," Belloc said in a statement. "Miscarriage rates increased where the father was over 35."

The findings are scheduled to be presented at the 24th annual conference of the European Society of Human Reproduction and Embryology in Barcelona, Spain.