Researchers say it becomes more difficult for men to father children as they age, especially if their female partner is older, too.

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Researchers say sperm count and lifestyle factors can lead to declining sperm quality in men over the age of 50.

For women, menopause is a signal that female fertility doesn’t last forever.

Men, on the other hand, constantly produce new sperm and some men past the age of 80 occasionally father children.

That fuels the myth that men remain fertile all of their lives and can parent children as long as they can perform sexually.

In reality, men’s fertility also may decline with age and, as a new study on older men taking part in in-vitro fertilization (IVF) shows, sometimes that decline is quite dramatic.

Researchers led by Dr. Guy Morris from the Centre for Reproductive and Genetic Health in London have concluded that IVF success rates decline significantly among men over age 51 — coincidentally, the same average age that women enter menopause.

The study of nearly 5,000 IVF and intracytoplasmic sperm injection (ICSI, where a single sperm is injected into an egg) attempts determined that 42 percent of men over age 51 met the World Health Organization’s standard for semen quality, compared to 61 percent of men under age 51.

In turn, researchers found that IVF/ICSI pregnancy rates declined as women aged. But they found that the same was also true of men.

Nearly half of male IVF/ICSI participants under age 35 in the study were able to successfully fertilize an egg. Among those ages 36 to 40, that rate fell to 42 percent and to 35 percent among men ages 41 to 45.

By ages 46 to 50, the success rate was down to 32 percent. And men over age 51 were able to facilitate pregnancy just 30 percent of the time.

“This drop-off will also be related to the maternal age and this decline in pregnancy rate becomes quite dramatic over 35 years for a woman,” Morris told Healthline. “Coupled with the effect of an older male partner, we were not too surprised by these rates.”

Like most couples, IVF/ICSI patients tend to be close in age. Among women, fertility begins to decline at age 32, studies have shown — well before most women reach menopause.

The study authors, who presented the findings at the recent annual meeting of the European Society of Reproduction and Embryology, described the results as “a public health message for men to not delay fatherhood.”

“Stories of celebrity men fathering children into their 60s may give a skewed perspective on the potential risks of delaying fatherhood,” said Morris. “Indeed, in natural conception and pregnancy it is only recently that evidence of risks associated with later fatherhood has become available. These more recent studies contrast with decades of evidence of the impact that maternal age has on fertility outcomes.”

Morris added, however, that “applying this to men outside of fertility treatment should be done with caution, as we were not studying that cohort.”

Morris speculated that declining semen quality related to aging could be the cause of decreased male fertility over age 51.

“Follow-up would ideally examine the underlying mechanism: Is it DNA fragmentation? And what interventions might be available to mitigate this decrease in the chance of success?” he said. “Could enhanced sperm selection techniques mitigate the decline in chance? Could simple lifestyle factors be of benefit?”

“Infertility is not just a female disease. Women and men are equals when it comes to infertility,” Corey Burke, tissue bank director at Cryos International, the world’s largest sperm bank and egg bank, told Healthline. “Men are also not immune to the various social risk factors that affect women, such as smoking, drinking, drugs, stress and more, and just as a woman’s egg quality decreases with age so does a man’s sperm quality.”

Many men, said Burke, “spend their youth doing stuff that’s not good for their reproductive health,” including such seemingly harmless activities such as combining a strenuous workout with time in the hot tub or wearing tight shorts while biking.

Anything that causes heat and compression on the testicles, including hours spent sitting and gaming, could have a long-term negative effect on male reproductive capacity, said Burke.

Even when older men successfully get a woman pregnant, such pregnancies can be higher risk — again, paralleling issues that are well known among women.

For example, studies have shown that children of older fathers are more likely to be born prematurely, or to have lower APGAR scores — a test routinely used to assess the health of newborns — said Dr. Michael Eisenberg, a urologist and director of male reproductive medicine and surgery at Stanford Health Care.

“If you look at both the father and mother, the mother has a very large impact, but the father’s impact is not negligible,” Eisenberg told Healthline. “He is donating half of the DNA.”

In addition to producing sperm with greater potential for mutation, older men also tend to have less semen volume and sperm with less motility, or ability to swim toward an egg, said Eisenberg.

“Usually these changes are small, but they’re definitely something to think about” if men want to father children at a later age,” he said.