A man's fertility appears to decline after the age of 40, in much the same way that a woman's ability to conceive fades after 35, according to French researchers.

Their study, of nearly 2,000 couples undergoing fertility treatment, found that pregnancy attempts were 70 percent more likely to fail when the man was age 40 or older than if he were younger than 30 -- regardless of his wife's age.

Because all the women in the study were completely sterile and undergoing in vitro fertilization (IVF), the age of the fathers was key. And while past studies have suggested that older men are less likely to father children, the extent to which this was related to biological changes or to decreased sexual activity has been unclear.

"Our results provide, for the first time, strong evidence for a paternal age effect on failure to conceive that is linked only to biological male aging," the study authors report in the journal Fertility and Sterility.

Dr. Elise de La Rochebrochard of the French national health institute INSERM led the study.

According to the researchers, the lower IVF success rate among relatively older men may be due to poorer-quality sperm.

It has long been known that women are less likely to conceive after the age of 35 than before, de La Rochebrochard and her colleagues note. But the current findings, they write, suggest that for men, the age of 40 is similarly important.

"In reproduction," the researchers conclude, "age must no longer be considered as the concern of the woman, but as that of the couple."

SOURCE: Fertility and Sterility, May 2006.