The Menstrual Cycle
Four phases, one feedback loop, and a monthly rehearsal for pregnancy
Evidence grade 5 of 5, Settled. Read how the Atlas grades evidence.
The physiology of the menstrual cycle is settled, textbook science: the hypothalamic-pituitary-ovarian feedback loop, the follicular, ovulatory, and luteal phases, and the FSH, LH, estradiol, and progesterone dynamics are described consistently across Endotext, the Merck Manual, and reproductive endocrinology references. Large cycle-tracking datasets (Soumpasis 2020, n=32,595 women, and Flo, n over 1.5 million) confirm the same physiology while showing that the popular 28-day cycle and day-14 ovulation are population averages, not rules.
Reference only. Not medical advice.
What can go wrong
Cycles that are very irregular, very heavy, extremely painful, or that stop unexpectedly can signal conditions that need evaluation, so the cycle is also a useful health signal.
This is something to take to a clinician. The Atlas can tell you what is known about the menstrual cycle in general. It cannot tell you what is happening in your body, and the difference matters most for exactly the symptoms that bring people to this page.
The cycle is a conversation between the brain and the ovaries that repeats roughly monthly, and knowing its four phases explains most of what changes across the month.
How it works
In the menstrual phase the lining sheds. In the follicular phase, rising follicle-stimulating hormone matures an egg while estrogen rebuilds the lining. A surge of luteinizing hormone triggers ovulation, releasing the egg. In the luteal phase, progesterone prepares the lining for a possible pregnancy, and if none occurs, hormones fall and the next period begins. This is a feedback loop, not a fixed clock.
What people get wrong
Often saidEvery cycle is 28 days.
ActuallyAnywhere from about 21 to 35 days is normal, and cycles naturally vary, especially in adolescence and perimenopause.
Often saidOvulation always happens on day 14.
ActuallyThe timing of ovulation varies from woman to woman and month to month, which is why fertility timing is not fixed.
Who profits, and who pushes back
Reproductive physiology
Describes the cycle as a feedback loop between the pituitary gland and the ovaries.
Cycle-tracking apps
Sells this. Has a commercial interest in a product sold as this compound, as of July 2026.
Market phase-based predictions that are useful for some purposes but often overstate day-by-day precision.
Where the research comes from
| Place | Share of published research |
|---|---|
| Universal | 100% |
How we got here
The words for month, moon, and menses share a Proto-Indo-European root meaning to measure, reflecting how cultures across history counted the cycle against the lunar month. Understanding of the mechanism came piece by piece across the twentieth century. In the 1920s Kyusaku Ogino in Japan and Hermann Knaus in Austria independently concluded that ovulation occurs about 12 to 16 days before the next period, which the Dutch physician Johannes Smulders turned into the calendar-based rhythm method around 1930. The individual reproductive hormones were then isolated and the full hypothalamic-pituitary-ovarian feedback loop mapped, and in the 2010s and 2020s large cycle-tracking datasets replaced the textbook 28-day, day-14 assumption with a picture of wide, normal variation between and within women.
Tracked against the moon for millennia and named for it in many languages, the cycle was decoded hormone by hormone across the twentieth century, which made both modern contraception and fertility treatment possible.
- Ancient worldCycle tracked against the lunar month
- 20th centuryIndividual reproductive hormones identified
- Late 20th centuryFull feedback loop understood
- 2010s-2020sCycle tracking becomes a consumer technology
How it is understood
A typical cycle runs about 21 to 35 days, and variation is normal, especially in the years after the first period and approaching menopause. The cycle is often described in four phases, each driven by a shifting balance of hormones.
At a glance
- Typical length
- About 21 to 35 days
- Phases
- Menstrual, follicular, ovulation, luteal
- Ovulation trigger
- Luteinizing hormone surge
- Luteal-phase hormone
- Progesterone
What this entry does not yet cover
What follows is unfinished work on our side, not a finding about The Menstrual Cycle. A short entry is not a safe entry.
- Cost and market. We have not documented what it sells for or who sells it.
- Supply chain. We have not mapped where it is made.
- Who uses it. We have not documented the user groups.
- What could change. No forward-looking summary yet.
- How it compares. We have not set it against the alternatives.
- Children and teenagers. We have not checked what is known about younger people.
- What people take it for. We have not catalogued the claims made for it.
Sources
Detailed HPO-axis physiology of the normal cycle: FSH-driven follicular recruitment, estradiol negative then positive feedback, the LH surge triggering ovulation, and progesterone-dominant luteal phase lasting about 13 to 15 days.
The Normal Menstrual Cycle and the Control of Ovulation, Endotext (NCBI Bookshelf)
Female reproductive endocrinology: GnRH pulses drive pituitary FSH and LH, ovarian estradiol and inhibin feedback, and the four-phase structure of the cycle.
Female Reproductive Endocrinology, Merck Manual Professional Edition
Cycle length varies widely and ovulation is not fixed to day 14: 87 percent of women had cycles of 23 to 35 days, only 12.4 percent actually had a 28-day cycle, and for 28-day cycles ovulation was most common on day 15 (27 percent), then day 16, then day 14 (20 percent), across a 10-day spread.
In a global cohort of over 1.5 million app users, only about 16 percent had a 28-day median cycle, confirming that the textbook 28-day cycle is uncommon.
The menstrual cycle should be treated as a vital sign of overall health; the statistically derived normal range in adolescents is roughly 21 to 45 days, and cycles outside it warrant evaluation.
Cycle length varies with age, race/ethnicity, and body mass index across a large US digital cohort, so a single population average understates real-world variation.
In the 1920s Kyusaku Ogino (Japan) and Hermann Knaus (Austria) independently concluded ovulation occurs about 12 to 16 days before the next period, the basis of the calendar-based rhythm method.
Calendar-based contraceptive methods, Wikipedia (with primary citations to Ogino 1923 and Knaus)
Etymology: menses and menstruation derive from Latin mensis (month), cognate with Greek mene (lunar month) and the roots of English month and moon, from a PIE root meaning to measure.
The HPO axis controls the cycle through GnRH pulse frequency, gonadotrope FSH/LH release, folliculogenesis, and ovarian steroid feedback loops.
The Hypothalamic-Pituitary-Ovarian Axis, Ovarian Disorders, and Brain Aging, PMC