Turns out women aren't the only ones with an expiration date on their fertility. An emerging body of research is showing that men, too, have a "biological clock."
Not only do men become less fecund as they age, but their fertility begins to decline relatively early around age 24, six years or so before women's. Historically, infertility has been seen as a female issue, as has the increased risk of Down syndrome and other birth defects, but studies now also link higher rates of autism, schizophrenia and Down syndrome in children born to older fathers. A recent paper by researchers at Sweden's Karolinska Institute found that the risk of bipolar disorder in children increased with paternal age, particularly in children born to men age 55 or older.
It used to be that "if you had hair on your chest, it was your wife's problem," says Barry Behr, an associate professor of obstetrics and gynecology at the Stanford Medical School and director of Stanford's in vitro fertilization laboratory. Even now, he said, though about half of infertility cases are caused by male factors, such as low sperm count or motility, there are many more tests to evaluate a woman's fertility than a man's.
To some degree, that bias is rooted in biology. Women are born with as many eggs as they'll ever have about a million. That number steadily diminishes, and "the best eggs are ovulated first," Behr says. The ones that remain after age 35 or so, on average are vulnerable to toxins, radiation and other insults that may degrade their quality and viability.
By contrast, men make new sperm about every 90 days, Behr says, so the logic has been that there should not be that much difference between a young man's sperm and an old man's. Indeed, men as old as 94 have been known to father children.
Still, the research suggests it gets harder with age. A French study published in the current issue of Reproductive BioMedicine Online found that in couples undergoing infertility treatment, the father's age had as much effect as the mother's on rates of pregnancy and miscarriage the older either parent was, the less likely they were to get pregnant, and the more likely to miscarry. Other studies have found similar trends: on average, it will take longer than a year to conceive for 8% of couples in which the man is younger than 25; that percentage nearly doubles, to 15%, in couples with men age 35 or older. Data have also suggested that couples whose partners are the same age, or in which the man is younger than the woman, are more likely to conceive within a year, compared with couples in which men are at least five years older than their partners.
There are many possible explanations for the decline in male fertility, from a decrease in the number of sperm and their motility to lower testosterone levels to the effects of other age-related diseases like diabetes, which is associated with erectile dysfunction and lower levels of testosterone. But researchers think that genetic factors may be behind the link between paternal age and a child's risk of bipolar disorder and psychiatric disorders like autism and schizophrenia, whose origins are increasingly being attributed to DNA. Although sperm may be no more than 90 days old, the cells that make sperm may be subject to increasing DNA mutations as men age, affecting the quality of the sperm they produce.
In the Swedish study, published Sept. 1 in the Archives of General Psychiatry, researchers found that the risk of developing bipolar disorder began to increase in children born to fathers around age 40. The highest risk, however, occurred in men age 55 and older; their offspring were 37% more likely to develop the disorder than children born to men in their 20s. Children of older men were also twice as likely to develop early-onset disease before age 20 which studies suggest has a strong genetic component.
What does all this mean for would-be older dads? While women are used to seeing grim statistics about their decreasing chances of achieving pregnancy and the increasing risks of Down syndrome as they age, men have typically believed that they have all the time in the world. Perhaps now, men in their mid-30s will start sharing the same "now or never" pressure to conceive that women have long endured.
When older men father children, Behr says, the traditional response has been to "pat them on the back and buy them a beer." He has seen patients that he felt were too old to become fathers, but "plenty of people make decisions about parenthood that I wouldn't," he says. "Our responsibility is to educate patients with the facts, and they decide."