The menstrual cycle runs through four phases — menstrual, follicular, ovulation, and luteal — driven by four hormones in sequence, and for adults a cycle anywhere from 24 to 38 days counts as normal, per ACOG's 2015 committee opinion on menstrual disorders in adolescents and adults. The phases are not four equal weeks: the first half of the cycle varies in length from person to person and month to month, while the second half is comparatively fixed, which is why cycle tracking works the way it does.
This article publishes information, not medical advice. It explains what happens in each phase, which hormones drive the changes, and where the wide edges of normal sit — useful context whether you track your cycle already or are trying to understand what a cycle is for. Decisions about symptoms belong with your own clinician.
What is the menstrual cycle, physiologically?
The menstrual cycle is the monthly sequence by which the ovary matures and releases an egg and the uterus prepares, then sheds, its lining. It is orchestrated by the hypothalamus and pituitary in the brain, which send signals to the ovaries, and by the ovarian hormones — estrogen and progesterone — that act on the uterus in return. The textbook average, 28 days, is genuinely just an average: ACOG's guidance treats intervals from 24 to 38 days as normal for adults, with even wider variation in the first years after periods begin and in the years approaching menopause.
What happens during the menstrual phase?
The cycle's visible phase: with progesterone and estrogen at their lowest, the uterine lining — built up over the previous weeks — sheds and exits as menstrual flow, lasting typically two to seven days. Hormone levels at this point are at their monthly low, which is why cramps, fatigue, and mood dips cluster here for many people. Cramps come from prostaglandins, hormone-like compounds that make the uterus contract to expel the lining; per ACOG, they are among the most common menstrual complaints and respond to nonsteroidal anti-inflammatory drugs such as ibuprofen for most people.
Volume is worth knowing in concrete terms: average total loss is roughly 30 to 40 milliliters across a period — a few tablespoons. ACOG's patient guidance flags heaviness worth reporting as soaking through a pad or tampon every hour for several hours, bleeding lasting longer than seven days, or periods that limit daily activities — patterns that can point to treatable causes such as iron deficiency or, per ACOG guidance, conditions like fibroids.
What happens during the follicular phase?
The follicular phase overlaps menstruation and continues after it: the pituitary releases follicle-stimulating hormone (FSH), which recruits a group of follicles — small fluid-filled sacs on the ovaries, each containing an immature egg. Usually one becomes dominant, and as it grows it produces rising amounts of estradiol, the main estrogen. Rising estrogen rebuilds the uterine lining and, near the phase's end, produces the physical signs many trackers note: clearer, stretchier cervical mucus and a small, documented rise in basal body temperature after ovulation follows, not before.
This is the variable half of the cycle. Stress, illness, travel, and sleep disruption can all lengthen it, which is why a cycle that drifts a few days from month to month is normal even in the same person. The follicular phase runs from the first day of bleeding until ovulation — day counts like "day 14" describe an average cycle, not a rule.
What happens during ovulation?
When estradiol peaks, the pituitary responds with a surge of luteinizing hormone (LH), which triggers the follicle to rupture and release the mature egg — ovulation, about 24 to 36 hours after the surge begins. The egg survives roughly 12 to 24 hours after release; sperm, by contrast, can survive in the reproductive tract up to about five days, per ACOG's fertility guidance. This asymmetry defines the fertile window: the days before ovulation, not the days after it, carry the pregnancy risk from a given act of intercourse.
Ovulation does not reliably occur on a fixed calendar day. Per ACOG, ovulation happens about 14 days before the next period, whatever the cycle's total length — so in a 35-day cycle it lands near day 21, not day 14. This single fact explains most surprises in natural family planning, in both directions.
What happens during the luteal phase?
After releasing its egg, the emptied follicle transforms into the corpus luteum, a temporary gland that produces progesterone, the hormone that stabilizes and enriches the uterine lining for a possible pregnancy. The luteal phase is comparatively fixed — typically about 12 to 14 days — and progesterone's rise and fall here drive what is often called the premenstrual window: breast tenderness, bloating, appetite changes, and mood shifts that resolve once bleeding begins. When the corpus luteum breaks down without a pregnancy, progesterone and estrogen fall together, the lining sheds, and the cycle restarts — the phases are one continuous loop, not separate events.
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How much variation is normal?
More than most people assume, per ACOG's 2015 committee opinion. Normal adult cycles run 24 to 38 days; cycles in the first two to three years after menarche commonly run longer and irregular because ovulation is not yet regular; and in perimenopause, intervals shorten, lengthen, and skip. Occasional variation of a few days is ordinary. What deserves clinical attention, per the same guidance: cycles consistently shorter than 24 or longer than 38 days; no period for three months without pregnancy, breastfeeding, or menopause; bleeding between periods or after sex; bleeding lasting more than seven days; or periods severe enough to miss work or school — a marker ACOG treats as a symptom to investigate, never a fact to normalize.
Does cycle tracking help?
As information, yes. A simple record — first day of bleeding, last day, heaviness, pain, notable symptoms — turns vague complaints into patterned data a clinician can use, per ACOG's guidance encouraging cycle awareness as a "vital sign" of reproductive health. Apps and ovulation predictor kits, which detect the LH surge in urine, add precision for people timing or avoiding pregnancy, though per ACOG none predicts ovulation perfectly, and app calendar predictions inherit the variability described above. Tracking is diagnostic context, not contraception by itself: fertility-awareness methods used for contraception have typical-use failure rates up to 23 percent per year, per CDC figures.
When to talk to a clinician
Bring cycles to a clinician, per the direction of ACOG's guidance, if periods stop for three months without an obvious reason; if cycles are consistently shorter than 24 or longer than 38 days; if bleeding lasts more than seven days or soaks through protection hourly; if bleeding occurs between periods, after sex, or after menopause; if pain requires missing school or work or does not respond to standard pain relievers; or if premenstrual symptoms regularly interfere with life, which can indicate premenstrual dysphoric disorder, a treatable condition per ACOG. Sudden severe pelvic pain warrants prompt, same-day evaluation.
Four phases, one loop: the follicular half varies, the luteal half holds steady, and the first day of bleeding is the cycle's true day one.
Frequently asked questions
Is a 28-day cycle required for normal fertility? No. ACOG's guidance defines normal adult cycles as anywhere from 24 to 38 days, and cycles at either end are compatible with regular ovulation. Length matters less than pattern: a cycle that is reliably 34 days for you is more informative than a cycle that swings between 26 and 36. Persistent irregularity, not a particular number, is what prompts evaluation.
Can I get pregnant right before my period? It is unlikely but not impossible. Pregnancy requires an egg, viable about 12 to 24 hours after ovulation, and sperm, which survive up to about five days, per ACOG. In a short or variable cycle, ovulation can arrive close to the expected period, which is why calendar-based assumptions fail. If avoiding pregnancy is the goal, no cycle day is treated as safe without a validated method.
Why do periods hurt more some months than others? Prostaglandin levels, the compounds that drive uterine contractions, vary between cycles, and pain perception varies with stress, sleep, and other conditions. Per ACOG, pain that responds to nonsteroidal anti-inflammatory drugs and heat, and does not worsen over time, is common. Pain that escalates, requires missed days, or arrives outside the bleeding window deserves evaluation for conditions such as endometriosis.
Do hormones really affect mood? Yes, in a documented, time-limited way. The premenstrual window's physical and mood symptoms track progesterone's fall in the late luteal phase, per ACOG. When symptoms are severe enough to disrupt work or relationships, ACOG recognizes premenstrual dysphoric disorder as a distinct, treatable condition — evidence that the connection is real without being deterministic, and that the severe end has clinical options.
What is the first day of the cycle? The first day of true menstrual flow — not spotting, not light brown discharge that precedes it — per standard clinical convention described in ACOG guidance. The definition matters for anything timed to the cycle: ovulation tests, fertility treatments, and progesterone testing all anchor to day one. Tracking apps apply the same rule, so entering the right first day keeps every later prediction honest.