Feb. 5, 2003 -- It turns out women aren't the only ones with biological clocks. Men have them, too, new research suggests, but instead of stopping abruptly, their clocks wind down gradually with age.
A study examining sperm quality among men from early adulthood to old age offers new evidence that fertility declines as males get older. The study found a 20% decline in semen volume among 50-year-olds compared with 30-year-olds, with similar declines seen for other semen quality measures.
"We found no evidence of an age threshold in men, like the one seen in women prior to menopause," researcher Brenda Eskenazi, PhD, tells WebMD. "What we saw was a gradual decline in semen quality over time."
The study was unique because it examined semen quality in healthy, nonsmoking men of different ages with no known fertility problems. Ninety-seven men between the ages of 22 and 80 provided semen samples for analysis and answered questions about lifestyle designed to identify risk factors for reduced fertility.
After adjusting for such lifestyle factors, Eskenazi and colleague Andrew Wyrobek, PhD, concluded that there are significant decreases in semen quality with age, and semen volume and sperm motility also decrease. Total progressive motility -- a measure of sperm's ability to move forward -- declines by almost 5% per year over the course of a man's reproductive life.
The researchers calculated that men have a 40% chance of having abnormal sperm motility at age 30 and a 80% chance by age 50. The probability rises to 100% for an 80-year-old man. The findings are reported Feb. 6 in the journal Human Reproduction.
"Because semen quality is a well-known proxy for fertility, this research suggests that men become progressively less fertile as they age," says Eskenazi, who is a professor of epidemiology and maternal and child health at UCLA Berkeley. "This has important implications for men who chose to delay fatherhood."
The study is not the first to link aging with reduced fertility among men. Research from the U.K., published in 2000, examined the time it took to achieve a pregnancy among more than 8,500 couples. That study concluded that the odds of conceiving in six months or less decrease by 2% annually every year that a man is over the age of 24.
Christopher Ford, BSc, Phd, who lead the research team, says that study also found a gradual decline in male fertility over time.
"This is a slow biological clock compared to a female's, but it does seem to exist," Ford tells WebMD. "The message is that we cannot ignore a man's age when assessing infertility."
Eskenazi says the age-related decline in male fertility may be caused by changes in the genital urinary tract or by repeated assaults on fertility accumulating over time.
"It is really hard to separate out the effects of age per se from the fact that as you age you have a greater opportunity for these [assaults]," she says. "We just don't know enough yet to say why this happens."