Women go through menopause because their ovaries run out of eggs. Every woman is born with a fixed supply of follicles (the tiny structures that house eggs), and once that supply drops below a critical threshold, the ovaries can no longer sustain a menstrual cycle. This typically happens between ages 45 and 55. But the full answer goes deeper than simple egg depletion: there are hormonal chain reactions that drive the transition, and compelling evolutionary reasons why human females evolved to stop reproducing decades before the end of their lives.
A Finite Supply of Eggs
The process starts before birth. At about 20 weeks of fetal development, a female fetus has roughly 7 million follicles in her ovaries. By the time she’s born, that number has already dropped to 1 to 2 million. By her first period, only about 300,000 to 400,000 remain. From puberty onward, roughly 800 to 900 follicles are recruited each month, though only one typically matures into an ovulated egg. The rest are reabsorbed by the body.
This lifelong drawdown eventually brings the follicle count to around 1,000. At that point, there simply aren’t enough follicles left to produce the hormones needed to maintain a regular cycle. The ovaries essentially retire, and menstruation stops for good. Unlike sperm production in men, which continues (albeit with declining quality) throughout life, the egg supply is never replenished. This biological clock is hardwired.
The Hormonal Chain Reaction
Menopause doesn’t happen overnight. The transition unfolds over years, driven by a cascade of hormonal shifts that starts well before periods actually stop.
As follicle numbers decline, the ovaries produce less of a protein called inhibin B, which normally keeps a brain hormone called FSH (follicle-stimulating hormone) in check. With that brake removed, FSH levels start climbing, sometimes years before a woman notices anything different about her cycle. The brain is essentially shouting louder at the ovaries to keep working, and the ovaries are less and less able to respond.
What’s surprising is that estrogen levels don’t drop right away. They actually stay relatively stable, or even rise slightly, well into the transition, likely because elevated FSH pushes the remaining follicles to work harder. It’s only in the final two years before the last period that estrogen begins a steep decline. The sharpest hormonal changes happen in a window of about 10 months before and two years after the final menstrual period. After that, both FSH and estrogen levels plateau at their new baseline.
This hormonal turbulence is why the perimenopausal years feel so unpredictable. Hormone levels don’t decline in a smooth, straight line. They swing wildly, sometimes day to day, which is why a single blood test during this stage is an unreliable snapshot of where a woman is in the process.
What Perimenopause Feels Like
The transition stage, called perimenopause, lasts about four years on average but can stretch to eight. Some women pass through it in just a few months. The first sign is usually irregular periods: cycles that come closer together or further apart, periods that are heavier or lighter than usual, or months skipped entirely.
Beyond the menstrual changes, the fluctuating hormones produce a wide range of symptoms. Hot flashes and night sweats are the most well-known, but mood shifts (irritability, depression, anxiety), sleep disruption, vaginal dryness, lower sex drive, and needing to urinate more frequently are all common. Not every woman experiences all of these, but most will deal with at least one. The intensity varies enormously from person to person.
Menopause itself is defined retrospectively: it’s the point at which you’ve gone 12 consecutive months without a period. Everything before that is perimenopause; everything after is postmenopause.
Why Evolution Favored It
Humans are unusual. Among primates, we’re one of very few species where females routinely live decades past their reproductive years. Chimpanzees and gorillas don’t do this. So why did natural selection preserve, or even favor, such a long post-reproductive life?
The most widely studied explanation is the grandmother hypothesis, published in a landmark paper in the Proceedings of the National Academy of Sciences. The idea is straightforward: aging females who stopped having their own children could invest energy in feeding and caring for their grandchildren. In hunter-gatherer societies, this help was critical. When a mother had a new baby and couldn’t forage as much, a grandmother could step in to feed the older, already-weaned children. This kept more grandchildren alive, which meant the grandmother’s genes, including those for a long post-reproductive lifespan, were passed on more successfully.
The evidence is striking. Human women produce babies at roughly double the rate of other great apes, and the grandmother hypothesis predicts exactly that: because grandmothers supplement the food supply, mothers can wean children earlier and have the next baby sooner. Even in hunter-gatherer populations with no access to modern medicine, adult women show distinctively low mortality rates compared to other apes, suggesting that long post-menopausal lifespans are not a modern artifact but an ancient, evolved trait.
A competing idea, sometimes called the mother hypothesis, takes a different angle. It proposes that menopause evolved because the risks of pregnancy and childbirth become dangerously high as the body ages. By stopping reproduction, a woman avoids dying in childbirth and can ensure her existing children survive to adulthood. However, studies in premodern human populations have found limited evidence supporting this version.
When It Happens Early
For some women, menopause arrives well ahead of schedule. Primary ovarian insufficiency (sometimes called premature menopause) is the loss of ovarian function before age 40. The causes include chromosomal differences such as Turner syndrome, damage from chemotherapy or radiation, autoimmune conditions where the body’s immune system attacks the ovaries, and certain genetic mutations. In many cases, no clear cause is ever identified.
Early menopause carries particular health concerns. Estrogen plays a protective role in maintaining bone density, and losing it earlier means more years of accelerated bone loss. In the five to seven years following menopause at any age, a woman can lose up to 20% of her bone density. When that clock starts at 35 instead of 50, the cumulative effect on skeletal health is significant.
What Changes in the Body Afterward
The drop in estrogen after menopause doesn’t just end periods. It reshapes several body systems. Bone loss is one of the most measurable consequences: the rapid decline in density during the first post-menopausal years is the primary reason osteoporosis disproportionately affects older women. Cardiovascular risk also shifts. Before menopause, estrogen helps maintain flexible blood vessels and favorable cholesterol levels. After menopause, heart disease risk gradually rises to match that of men.
There’s also a nuanced relationship between menopause and cancer. The cessation of ovulation means the ovaries are no longer undergoing the repeated cycle of rupture and repair that some researchers believe increases the chance of harmful mutations over time. However, earlier menopause does not appear to be protective against gynecological cancers. In fact, research using large national health datasets has found the opposite: women who went through menopause at a younger age had a higher prevalence of both cervical and ovarian cancer. Each additional year of age at menopause was associated with roughly a 9 to 10% lower odds of these cancers.
Menopause is not a disease or a malfunction. It’s a deeply embedded biological transition shaped by millions of years of natural selection, driven by a finite egg supply, orchestrated by a complex hormonal feedback loop, and, if the evolutionary evidence holds, ultimately beneficial to the survival of the broader family unit.