It turns out that men have a biological clock, too. It just ticks differently than women's.

For women in their 30s, the ticking of the biological clock has long provided fodder for magazine articles, books and even jokes. But men have usually had a free ride when it comes to fertility. New research, however, has an implication that is only starting to be understood: Now, it is the man's turn to worry.

The latest studies show that the amount of semen, its quality, as well as the speed at which it is able to move toward the egg, all gradually decline with age. In addition, with technological advances making it possible for scientists to study the behavior of a single sperm, many doctors say it is clear that a man's role in determining a miscarriage or the health of the unborn child is far larger than once thought.

One troubling recent finding for would-be fathers: Men 35 or older have half the chance of fathering a child within 12 months, compared with a man who is younger than 25. That is the case even when the age of the mother is taken into account. As a result, men are focusing more than ever on maintaining high testosterone levels in hopes of keeping sperm production high. Others are taking advantage of new tests that measure the level of genetic damage in sperm.

All this is changing the focus of the nation's fertility industry. "This isn't a women-only issue anymore," says Andrew Wyrobek, director of the health effects genetics division at the Lawrence Livermore National Laboratory in Livermore, Calif., where new tests have been developed that allow scientists to detect abnormal chromosomes in sperm.

With the number of men aged 35 to 54 who are fathering children up nearly 25% in the past 20 years, clinics that focus on male reproductive health are springing up all over the country.

Top practices such as Reproductive Medicine Associates of New York, a Manhattan fertility clinic, now tout their ability to coordinate couples' treatment by having male and female fertility specialists on staff.

The upshot is that relatively young men who hadn't given a second thought to issues such as sperm production are suddenly being forced to develop new medical expertise. Harold Bafitis never expected to have difficulties becoming a father when he got married. A fertility specialist, however, told him that his sperm weren't moving as quickly as they used to when he was younger. "I had never given any thought that there might be a biological clock for men," says Dr. Bafitis, a plastic surgeon in West Palm Beach, Fla.

After surgery to tie off some dilated veins around his testicles, which helped make his sperm speedier, he became the father of twins at the age of 43. Now he is hoping to slow down his clock. Today, at 49, he gets hormone injections twice a week designed to boost his body's natural sperm production and testosterone levels.

This new focus on male fertility has created a booming market for testosterone products such as hormone patches and gels, with more expected in pharmacies in the next few years. Sales of testosterone-replacement products went from $100 million in 2000 to over $350 million last year and are expected to exceed $1 billion in the next five years, according to Frost & Sullivan, a consulting firm in San Jose, Calif.

"This is similar to what happened in the market for cholesterol products," says Ajit Baid, an industry manager for pharmaceuticals for the firm. Despite the interest, doctors still caution that more long-term study of hormone therapy is needed.

Crucial Factor

Personal choices play a role, but age is turning out to be a crucial factor: Studies show that men who smoke a pack of cigarettes a day have twice the number of abnormal sperm as non-smokers and that alcohol, lead, and chemotherapy treatments for cancer can also affectsperm volume, speed or shape.
But even in a healthy 50-year-old man, semen volume could be 20% lower than it was when he was 30 years old. A sperm's motility -- the ability of the sperm to swim quickly toward the egg -- is around 30% slower. The number of abnormally shaped sperm is higher in 50-year-old men, raising the chances of fathering a child with a birth defect.

Unlike women, who typically stop ovulating in their 40s, men of course can still father children for decades longer, and there is no known age when sperm production dramatically drops off. Indeed, many doctors remain leery of the term "male biological clock" since it works so differently than women's fertility clock. "It doesn't strike midnight and everything stops," says Larry Lipshultz, a professor of urology at Baylor College of Medicine in Houston. "It's more like a gradual winding down."

Despite proud papas such as actor Tony Randall, who fathered children in his 70s, it is increasingly clear that being fertile at one time in your life is no guarantee you will be later on. George Hacher, a 72-year-old entrepreneur from Laredo, Texas, has five children and six grandchildren from a first marriage. When his second wife, 36, said she wanted children, he figured it would happen as quickly as his first attempts: "I felt I was going to hang my pants on the end of the bed and 'Bingo!'" says Mr. Hacher.

Instead, the couple had difficulty getting pregnant. When Mr. Hacher went in for tests, he was told that his sperm production had declined, so doctors extracted some sperm from his testicles and banked them. He and his wife are going to try to have a child using in vitro fertilization.

Scientific advances are making it easier than ever for men to find out about their sperm. Now Livermore and other labs use chemically labeled DNA probes in the sperm head to rapidly analyze thousands of cells inside the sperm. The studies show that sperm not only fertilize the egg, but help determine whether the resulting pregnancy will survive to birth. Men who are carriers of the gene for cystic fibrosis, for example, father pregnancies that have miscarriage rates as high as 35%, compared with a 10% miscarriage rate in the general population.

For men who want to find out more, new tests now "go beyond the standard semen analysis," says Natan Bar-Chama, the director of the Center for Male Reproductive Health at Reproductive Medicine Associates of New York. The sperm chromatin structure test and the sperm DNA integrity test, which measure the level of genetic damage inside the sperm chromosomes, cost $200-$300 and are increasingly being used by fertility clinics. Men who are older show higher levels of damaged sperm, which may affect their potential to father children.

To Freeze or Not to Freeze

For most men, even those who think they will not marry or father children until later in life, storing sperm in a sperm bank probably isn't necessary. "It's better to use fresh sperm from a 45-year-old man who doesn't smoke and isn't overweight than a frozen sample from a younger man that is thawed," says Dr. Bar-Chama.

Still, even the most diehard enthusiasts of the new fertility regimens realize there are limits. "You may be able to turn back the clock," says Dr. Bafitis, the plastic surgeon, "but it still keeps ticking."