Human ovaries were tuned for late menarche, long lactation and roughly a hundred lifetime menses. Modern early puberty, few births and short breastfeeding ask them to ovulate three to four times as often. Roger Short, Beverly Strassmann and S. Boyd Eaton mapped the mismatch.
A monthly period feels like the default setting of adult female life. Textbooks draw a 28-day wheel. Calendars mark it. Apps count it. The implication is that a woman who is not pregnant should bleed, more or less on schedule, from the early teens until the early fifties.
That calendar is historically new.
The ovaries still expect a different career: later first bleeding, years spent pregnant or in lactational amenorrhea, and a lifetime total of menses measured in the low hundreds rather than four hundred. The tissue is the same. The number of times it is asked to grow a follicle, surge luteinizing hormone, rupture an ovary surface, and rebuild a thick endometrium is not.
The mismatch is not a moral argument about family size. It is a counting argument about how often a pair of glands is asked to run a program they evolved to run far less often.
What the ovaries actually do each month
An ovary is not a passive egg warehouse. It is a steroid factory wrapped around a finite pool of follicles.
Each cycle a cohort of follicles is recruited. One usually becomes dominant. Granulosa cells aromatize androgens into estradiol. Estradiol thickens the endometrium and, near mid-cycle, trips the pituitary LH surge. The dominant follicle ruptures. The remnant becomes a corpus luteum and secretes progesterone for about two weeks if no pregnancy implants. If none does, the corpus luteum fails, progesterone falls, and the lining sheds.
That sequence is metabolically expensive. It is also inflammatory. Ovulation is a controlled wound on the ovarian surface. Menstruation is a controlled demolition of the uterine lining. Both leave debris, cytokines, and a need to repair.
Do that a hundred times and the bookkeeping is one thing. Do it four hundred times and the same bookkeeping has a different remainder. That remainder shows up, in susceptible people, as endometriosis risk, heavier cumulative estrogen exposure to breast and endometrium, and a different perimenopausal landing.
The ancestral tally is not a guess
In the 1970s the reproductive biologist Roger Short pointed out what textbooks had ignored: in natural-fertility populations, women spend most of reproductive life either pregnant or not cycling because they are breastfeeding. Monthly menstruation is not the ancestral monthly event. It is the event that happens in the gaps.
S. Boyd Eaton and colleagues later estimated that a hunter-gatherer woman who reached menopause might ovulate on the order of 160 times. That number was a model, not a census.
Beverly Strassmann supplied the census. Working among the Dogon of Mali, a natural-fertility farming population without contraception, she recorded menstrual-hut attendance night after night for years and checked the record against urinary hormones. After correction for missed hut visits, Dogon women who survived to menopause had a mean of about 128 lifetime menses and a median near 109. Menarche sat around 16. Menopause sat around 50. Between those poles, pregnancy and long, intensive breastfeeding emptied most of the calendar. From ages 20 to 34, the average woman had only slightly more than one period a year.
North American and European women, by contrast, now typically reach menarche near 12, have one or two children, breastfeed for months rather than years, and accumulate on the order of 350 to 450 ovulatory cycles if they do not use hormonal contraception. Eaton put the modern figure near 450. Later syntheses for high-income countries cluster around 400 to 460 lifetime ovulatory menstruations.
That is not a small shift. It is a three- to four-fold increase in the number of times the ovarian surface is wounded and the endometrium is built and torn down.
Why the number of cycles changed
Four modern facts did most of the work.
Earlier menarche. Better childhood nutrition, higher body fat, and less physically demanding girlhood pulled first bleeding earlier by three to four years compared with many 19th-century and subsistence populations. Those extra teenage years are almost all cycling years.
Fewer births. Demographic transition dropped completed fertility from five to ten live births toward one or two in much of the world. Each pregnancy is a long pause. Removing most of those pauses fills the calendar with cycles.
Shorter, less intensive lactation. Exclusive, frequent, night-inclusive breastfeeding is a powerful suppressor of the hypothalamic pulse generator that drives FSH and LH. Pumping, scheduled feeds, early complementary food, and shorter total duration all weaken that suppression. The ovary returns to work sooner.
Contraception that still allows withdrawal bleeding. Combined hormonal contraceptives prevent ovulation, which is a different and important story. Many users still have scheduled bleeds that are not true periods. The lifetime-ovulation math changes for pill users. The cultural idea that monthly bleeding is mandatory does not.
None of these changes required a mutation in the ovary. They required a change in the environment the ovary reports to.
What extra cycles cost a tissue that expected fewer
The cost is not that menstruation is a disease. It is that repeated ovulation and menstruation are exposures.
Endometriosis and adenomyosis. Paolo Vercellini and colleagues have argued that early-onset disease tracks, in susceptible women, with the number of ovulatory menstruations in the years after menarche — especially the long interval before first full-term pregnancy. Stacey Missmer’s Nurses’ Health Study analyses found higher endometriosis risk in the highest quartile of lifetime ovulatory cycles among never-users of combined oral contraceptives. Retrograde menstruation is common. Hundreds of episodes give more chances for ectopic endometrium to seed and persist. This is association and mechanism, not destiny. Most high-cycle women never develop endometriosis. The dose-response still matters for those who do.
Breast and endometrium. Cumulative estrogen-plus-progesterone exposure from repeated ovulatory cycles is part of the epidemiologic story of breast and endometrial cancer risk, alongside age at menarche, age at first birth, parity, and lactation. Malcolm Pike and others framed this decades ago as a problem of extra mitotic seasons in tissues that ancestral life interrupted with pregnancy and amenorrhea. Pregnancy and breastfeeding are not anti-cancer talismans. They are long interruptions in a proliferative program.
Ovarian surface. Each ovulation is a repair event. The incessant-ovulation hypothesis, associated with work following Fathalla, treats repeated rupture and repair as one contributor to epithelial ovarian cancer risk. Pregnancy and oral contraceptives, which quiet ovulation, associate with lower risk. The hypothesis is not the whole disease. It is one reason the modern cycle count is not biologically free.
Cramps as a monthly tax. Primary dysmenorrhea is prostaglandin work in a lining that has to come off. More cycles means more months of that tax for women who are susceptible, a pattern already described in why first-day period cramps are so common.
None of this means “have more children for your ovaries.” It means the modern default of hundreds of unopposed cycles is a novel load on organs that were usually busy with gestation or quiet under prolactin.
The other mismatch: cycles that look regular but do not ovulate
Jerilynn Prior’s work at the Centre for Menstrual Cycle and Ovulation Research complicates the tidy 28-day cartoon from the other direction.
A bleed is not proof of ovulation. Normal-length cycles can be anovulatory or have a short luteal phase, which means estrogen without an adequate progesterone sequel. Prior’s prospective work has found subclinical ovulatory disturbances in a substantial fraction of normal-length cycles in young urban women. Stress, under-fueling, and over-training all push the hypothalamus toward that pattern.
So modern ovaries are asked to cycle more often — and, in a subset of months, they cycle without finishing the progesterone half of the script. The first mismatch is extra ovulations across a lifetime. The second is extra estrogen-dominant months inside that lifetime. Both sit inside the larger pattern of why women’s bodies pay a higher price for modern living.
The fertility calendar itself has also been pulled off ancestral timing, a theme unpacked in when fertility met the modern calendar.
What this is not
It is not an argument that monthly periods are unnatural in the sense of fake. They are a real, healthy option when pregnancy is not the goal.
It is not an argument that every woman should use continuous hormonal suppression. Suppression has benefits and trade-offs that belong to an individual and a clinician, not to an evolutionary essay.
It is not a claim that Dogon or forager life is a template to copy. Child mortality, workload, and limited contraception are not nostalgia.
It is a claim about design load. The ovary and endometrium were built to spend long stretches off the monthly clock. Modern life put them on the clock early and left them there.
How to treat the glands as if the old math still matters
You cannot put menarche back to 16 on purpose, and you should not try. You can stop treating monthly demolition as the only respectable setting.
Know whether you ovulate, not only whether you bleed. Cycle length is a start. Mid-luteal progesterone, urinary LH kits, or carefully read basal temperature can show whether the second half of the script ran. Prior’s point is practical: a monthly bleed can hide a missing ovulation.
Do not confuse scheduled pill bleeds with periods. Withdrawal bleeding is useful for some people and psychologically grounding for others. It is not proof the ovary did its ancestral job that month. Continuous or extended regimens are medically legitimate options when the goal is fewer endometrial builds.
Protect the teenage ovary from a false emergency. Early combined-pill prescriptions for acne or cramps are common. They can be the right tool. They are not the only tool, and they are not a requirement of a “normal” adolescence. The first years after menarche are often irregular while the axis matures. That irregularity is frequently physiology, not a diagnosis that must be overwritten.
Take pelvic pain seriously early. If extra cycles raise the opportunity for endometriosis in susceptible people, waiting years with “normal period pain” is the opposite of matching the modern load. Pain that stops school, sport, or sleep deserves evaluation, not endurance coaching.
Lactation is not a wellness accessory. When a woman wants to breastfeed and can, frequent, including-night feeding is the signal the hypothalamus evolved to read. Support for that signal is reproductive physiology, not lifestyle branding.
Fuel the cycle you do have. Under-eating plus high training volume is a reliable way to lose ovulation while keeping a bleed or losing the bleed entirely. The ovary notices energy availability faster than a calendar app does.
When to see a doctor
Seek care rather than evolutionary explanation if:
- Periods are so painful that they regularly stop work, school, or sport.
- Bleeding soaks through protection hourly, lasts longer than a week, or arrives after menopause.
- Cycles suddenly vanish for three months or more and you are not pregnant, not fully breastfeeding, and not on suppression.
- You have been trying to conceive for 12 months (six if 35 or older) without success.
- There is pain with intercourse, bowel movements during menses, or unexplained infertility — the cluster that should raise endometriosis on the list.
- You have heavy bleeding plus dizziness, shortness of breath, or known anemia.
Those are clinical problems. The cycle-count story explains why some of them became more common. It does not diagnose any one person.
Myths vs facts
Myth: A period every month is what female bodies were designed to do.
Fact: In natural-fertility life, long stretches of pregnancy and lactational amenorrhea were the usual adult pattern. Monthly cycling was the gap, not the ground state.
Myth: More periods means a healthier, more fertile system.
Fact: Fertility lives in ovulatory quality and timing, not in the raw number of bleeds. Extra lifetime ovulations are an exposure, not a vitality score.
Myth: If you bleed on a combined pill, you ovulated.
Fact: Combined pills suppress ovulation. The bleed is withdrawal from the hormones in the pack.
Myth: Irregular cycles in the first years after menarche always need immediate hormonal regulation.
Fact: The axis often takes time to settle. Persistent pain, hemorrhage, or very infrequent cycles still need a workup. Mild teen irregularity often does not.
Myth: Evolutionary mismatch means you should reject modern contraception.
Fact: Contraception is one of the tools that lets people choose cycle number. The mismatch describes the default load without it, not a duty to refuse it.
FAQs
How many periods did women used to have?
In Strassmann’s Dogon data, on the order of 100 to 130 lifetime menses after correction. Models for hunter-gatherers sit in a similar low-hundreds range. Modern high-income figures without hormonal suppression cluster near 400.
Does that mean menstruation is unhealthy?
No. A period is a normal way to shed a lining that was prepared for a pregnancy that did not happen. The health question is cumulative dose: how many times the wound-and-rebuild program runs.
Do birth control pills fix the mismatch?
They reduce ovulations, which is the exposure several of the risk stories care about. They introduce other hormone patterns. Whether that trade is net helpful is individual — migraine history, clot risk, bone, mood, cancer family history, and preference all belong in the decision.
Can lifestyle put the ovary “back” on an ancestral schedule?
Not fully, and not safely as a project. You can support ovulation quality with food, sleep, and sane training. You cannot ethically recreate late menarche and eight births as a wellness protocol.
Why do some athletes lose their periods?
Low energy availability suppresses the hypothalamic GnRH pulse generator. The ovary is downstream. That is a different mismatch — not enough fuel — stacked on the cycle-count mismatch.
Is endometriosis caused by too many periods?
Not by itself. Retrograde flow is common. Genetics, immune clearance, and local inflammation decide who gets disease. More ovulatory menstruations give more opportunities. That is why early, severe pain deserves investigation rather than a decade of “normal cramps.”
Conclusion
The ovary is a finite, cycling organ that evolved to spend much of adult life not cycling. Late menarche, repeated pregnancy, and long lactation used to write long quiet stretches into the record. Modern nutrition, small families, and short breastfeeding erased most of those stretches.
The glands did not get a memo. They still recruit follicles, surge, rupture, and rebuild as if the next pregnancy might be soon and the next quiet year might be long. What they get instead, for many women, is a few hundred extra performances of the same play.
You do not owe your ovaries a forager biography. You do owe them an honest count. Monthly bleeding is common now. It was never the only setting the system knew, and it is not the only setting it can use.
