Medical News

A Fundamental Stage of Human Reproduction Is Shifting

For a long time, having children has been a young person’s game. Although ancient records are sparse, researchers estimate that, for most of human history, women most typically conceived their first child in their late teens or early 20s and stopped having kids shortly thereafter.

But in recent decades, people around the world, especially in wealthy, developed countries, have been starting their families later and later. Since the 1970s, American women have on average delayed the beginning of parenthood from age 21 to 27; Korean women have nudged the number past 32. As more women have kids in their 40s, the average age at which women give birth to any of their kids is now above 30, or fast approaching it, in most high-income nations.

Rama Singh, an evolutionary biologist at McMaster University, in Canada, thinks that if women keep having babies later in life, another fundamental reproductive stage could change: Women might start to enter menopause later too. That age currently sits around 50, a figure that some researchers believe has held since the genesis of our species. But to Singh’s mind, no ironclad biological law is stopping women’s reproductive years from stretching far past that threshold. If women decide to keep having kids at older ages, he told me, one day, hundreds of thousands of years from now, menopause could—theoretically—entirely disappear.

Singh’s viewpoint is not mainstream in his field. But shifts in human childbearing behavior aren’t the only reason that menopause may be on the move. Humans are, on the whole, living longer now, and are in several ways healthier than our ancient ancestors. And in the past few decades, especially, researchers have made technological leaps that enable them to tinker like never before with how people’s bodies function and age. All of these factors might well combine to alter menopause’s timeline. It’s a grand experiment in human reproduction, and scientists don’t yet know what the result might be.

So far, scientists have only scant evidence that the age of onset for menopause has begun to drift. Just a few studies, mostly tracking trends from recent decades, have noted a shift on the order of a year or two among women in certain Western countries, including the U.S. and Finland. Singh, though, thinks that could be just the start. Menopause can come on anywhere from a person’s 30s to their 60s, and the timing appears to be heavily influenced by genetics. That variation suggests some evolutionary wiggle room. If healthy kids keep being born to older and older parents, “I could see the age of menopause getting later,” Megan Arnot, an anthropologist at University College London, told me.

Singh’s idea assumes that menopause is not necessary for humans—or any animal, for that matter—to survive. And if a species’ primary directive is to perpetuate itself, a lifespan that substantially exceeds fertility does seem paradoxical. Researchers have found lengthy post-reproductive lifespans in only a handful of other creatures—among them, five species of toothed whales, plus a single population of wild chimpanzees. But women consistently spend a third to half of their life in menopause, the most documented in any mammal.

In humans, menopause occurs around the time when ovaries contain fewer than about 1,000 eggs, at which point ovulation halts and bodywide levels of hormones such as estrogen plummet. But there’s no biological imperative for female reproductive capacity to flame out after five decades of life. Each human woman is born with some 1 to 2 million eggs—comparable to what researchers have estimated in elephants, which remain fertile well into their 60s and 70s. Nor do animal eggs appear to have a built-in expiration date: Certain whales, for instance, have been documented bearing offspring past the age of 100.

[Read: Why killer whales (and humans) go through menopause]

This disconnect has led some researchers to conclude that menopause is an unfortunate evolutionary accident. Maybe, as some have argued, menopause is a by-product of long lifespans evolving so quickly that the ovaries didn’t catch up. But many women have survived well past menopause for the bulk of human history. Singh contends that menopause is a side effect of men preferring to mate with younger women, allowing fertility-compromising mutations to accumulate in aged females. (Had women been the ones to seek out only younger men, he told me, men would have evolved their own version of menopause.) Others disagree: Arnot told me that, if anything, many of today’s men may prefer younger women because fertility declines with age, rather than the other way around.

But the preponderance of evidence supports menopause being beneficial to the species it’s evolved in, including us, Francisco Úbeda de Torres, a mathematical biologist at Royal Holloway, University of London, told me. Certainly, menopause was important enough that it appears to have arisen multiple times—at least four separate times among whales alone, Samuel Ellis, a biologist at the University of Exeter, told me.

One of the most prominent and well-backed ideas about why revolves around grandmothering. Maybe menopause evolved to rid older women of the burden of fertility, freeing up their time and energy to allow them to help their offspring raise their own needy kids. In human populations around the world, grandmother input has clearly boosted the survival of younger generations; the same appears to be true among orcas and other toothed whales. Kristen Hawkes, an anthropologist at the University of Utah, argues that the influence of menopausal grandmothering was so immense that it helped us grow bigger brains and shaped the family structures that still govern modern societies; it is, she told me, sufficient to explain menopause in humans, and what has made us the people we are today.

[From the October 2019 issue: The secret power of menopause]

Some researchers suspect that menopause may have other perks. Kevin Langergraber, an ecologist at Arizona State University, points out that certain populations of chimpanzees can also live well past menopause, even though their species doesn’t really grandmother at all. In chimpanzees and some other animals, he told me, menopause might help reduce the competition for resources between mothers and their children as they simultaneously try to raise young offspring.

Regardless of the precise reasons, menopause may be deeply ingrained in our lineage—so much so that it could be difficult to adjust or undo. After all this time of living with an early end to ovulation, there is probably “no single master time-giver” switch that could be flipped to simply extend human female fertility, Michael Cant, an evolutionary biologist at the University of Exeter, told me.

Perhaps, though, menopause’s timeline could still change—not on scales of hundreds of thousands of years, but within generations. Malnutrition and smoking, for instance, are linked to an early sunsetting of menses, while contraceptive use may push the age of menopause onset back—potentially because of the ways in which these factors can affect hormones. Menopause also tends to occur earlier among women of lower socioeconomic status and with less education. Accordingly, interventions as simple as improving childhood nutrition might be enough to raise the average start of menopause in certain parts of the world, Lynnette Sievert, an anthropologist at the University of Massachusetts at Amherst, told me.

[Read: Why so many accidental pregnancies happen in your 40s]

Changes such as those would likely operate mostly on the margins—perhaps closing some of the gaps between poorer and richer nations, which can span about five years. Bigger shifts, experts told me, would probably require medical innovation that can slow, halt, or even reverse the premature aging of the ovaries, and maintain a person’s prior levels of estrogen and other reproductive hormones. Kara Goldman, an obstetrician-gynecologist and a reproductive scientist at Northwestern University, told me that one key to the ovarian fountain of youth might be finding drugs to preserve the structures that house immature eggs in a kind of dormant early state. Other researchers see promise in rejuvenating the tissues that maintain eggs in a healthy state. Still others are generating cells and hormones in the lab in an attempt to supplement what the aging female body naturally loses. Deena Emera, an evolutionary geneticist at the Buck Institute for Research on Aging, in California, thinks some of the best inspiration could come from species that stay fertile very late into life. Bowhead whales, for instance, can reproduce past the age of 100—and don’t seem to succumb to cancer. Maybe, Emera told me, they’re especially good at repairing DNA damage in reproductive and nonreproductive cells alike.

Some women may welcome an extended interval in which to consider having kids, but Goldman and Emera are most focused on minimizing menopause’s health costs. Studies have repeatedly linked the menopause-related drop in hormones to declines in bone health; some research has pointed to cardiovascular and cognitive issues as well. Entering menopause can entail years of symptoms such as hot flashes, urinary incontinence, vaginal dryness, insomnia, and low libido. Putting all of that off, perhaps indefinitely, could extend the period in which women live healthfully, buoyed by their reproductive hormones.

[Read: Women in menopause are getting short shrift]

Extending the ovaries’ shelf life won’t necessarily reverse or even mitigate menopause’s unwanted effects, Stephanie Faubion, the director of Mayo Clinic’s Center for Women’s Health, told me. Plus, it may come with additional risks related to later-in-life pregnancies. It could also raise a woman’s chances of breast or uterine cancer, blood clots, and stroke, Jerilynn Prior, an endocrinologist at the University of British Columbia, told me. And putting off menopause may also mean more years of menstruation and contraception, a prospect that will likely give many women pause, says Nanette Santoro, an obstetrician-gynecologist and a reproductive scientist at the University of Colorado School of Medicine.

But several researchers think some tweaking is worth a shot. Even if menopause once helped our species survive, Goldman said, “it’s hard to imagine” that’s still the case. Evolution may have saddled us with an odd misalignment in the lifespans of the ovaries and the other organs they live alongside. But it has also equipped us with the smarts to potentially break free of those limits.