The menstrual cycle is a repeating sequence of hormonal signals between the brain and ovaries that prepares the body for a possible pregnancy each month. It unfolds in four phases: menstrual, follicular, ovulatory, and luteal. Each phase has a distinct hormonal profile and a specific job, from shedding the uterine lining to selecting an egg to building a nutrient-rich environment for potential implantation. The whole process typically spans about 28 days, though anywhere from 21 to 35 days is considered normal, and it involves far more of the body than just the uterus.
The Menstrual Phase
The cycle begins with menstruation, which is both an ending and a beginning. When pregnancy hasn’t occurred, levels of estrogen and progesterone drop sharply. That hormonal withdrawal triggers the uterine lining, called the endometrium, to break down and shed. Menstrual bleeding typically lasts three to seven days and involves a surprisingly complex set of events happening simultaneously: inflammation, tissue breakdown, blood vessel constriction to limit bleeding, and the early stages of repair and regrowth that will carry over into the next phase.1PubMed Central. Uterine bleeding: how understanding endometrial physiology underpins menstrual health
That interplay between destruction and repair is why period bleeding is different from a wound. The endometrium doesn’t just passively fall away; the body is actively controlling how much tissue is shed, sealing off exposed blood vessels, and kickstarting regeneration while you’re still bleeding. Most people lose roughly 30 to 80 milliliters of blood per period, though it often looks like more because menstrual fluid also contains tissue and mucus.
The Follicular Phase
The follicular phase overlaps with menstruation at first and extends beyond it, typically lasting about 10 to 14 days. Its defining event is the selection and growth of one egg-containing follicle in the ovary. At the start of the cycle, the brain’s pituitary gland releases follicle-stimulating hormone (FSH), which nudges a small group of ovarian follicles to begin developing. Each follicle contains an immature egg surrounded by supporting cells.
Not all of these follicles survive. As the developing follicles begin producing estrogen, that estrogen feeds back to the pituitary and brings FSH levels down. The drop in FSH is essentially a selection pressure: most follicles need a certain threshold of FSH to keep growing, and as levels fall, only the strongest one, the “dominant follicle,” can continue.2PubMed. Mechanisms of follicle selection and development The dominant follicle manages this by becoming increasingly sensitive to whatever FSH remains, and eventually shifting to depend on a different hormone, luteinizing hormone (LH), for its final maturation. The rest of the follicles in the original group are reabsorbed by the body in a process called atresia.3Biology of Reproduction. Ovarian Follicular Growth and Development in Mammals
While this competition plays out in the ovary, the rising estrogen from the growing follicles is doing critical work in the uterus. Estrogen drives the endometrium to rebuild, stimulating cell proliferation and thickening the lining that was just shed during menstruation.4PubMed Central. Estrogen Receptor Function: Impact on the Human Endometrium Estrogen also prompts the endometrial cells to produce progesterone receptors, essentially priming the tissue to respond to progesterone later in the cycle.5Human Reproduction Update. The pathophysiological role of estrogens in the initial stages of pregnancy: molecular mechanisms and clinical implications for pregnancy outcome from the periconceptional period to end of the first trimester Without this groundwork, the luteal phase wouldn’t function properly.
Ovulation
Ovulation is the shortest phase, lasting roughly 12 to 24 hours, and it hinges on a dramatic hormonal reversal. Throughout most of the follicular phase, estrogen suppresses the release of LH from the pituitary. But when estrogen from the dominant follicle reaches a sustained high level, the brain flips the script: instead of suppression, estrogen triggers a massive surge of gonadotropin-releasing hormone (GnRH) from the hypothalamus, which in turn causes a spike in LH from the pituitary.6PubMed Central. Estrogen positive feedback to gonadotropin-releasing hormone (GnRH) neurons in the rodent: the case for the rostral periventricular area of the third ventricle (RP3V) This switch from negative to positive feedback is one of the most striking features of the cycle.7PubMed Central. The neurobiology of preovulatory and estradiol-induced gonadotropin-releasing hormone surges
The LH surge causes the dominant follicle to rupture, releasing the mature egg into the fallopian tube. The egg is viable for about 12 to 24 hours after release. Some people feel a twinge of pain on one side during ovulation, sometimes called mittelschmerz, though many notice nothing at all. Ovulation is also the event that home ovulation-predictor kits detect: they measure the spike in LH in urine, which typically precedes egg release by about 24 to 36 hours.
The Luteal Phase
After ovulation, the ruptured follicle left behind on the ovary transforms into a temporary structure called the corpus luteum. This small gland pumps out large amounts of progesterone, along with some estrogen.8PubMed Central. The inadequate corpus luteum The corpus luteum is genuinely a temporary endocrine organ with a built-in expiration date, and its progesterone output is central to everything that happens during this phase.9PubMed. Human corpus luteum physiology and the luteal-phase dysfunction associated with ovarian stimulation
Progesterone transforms the uterine lining from a proliferating tissue into a secretory one, ready to support a fertilized egg. The endometrial glands begin producing nutrients and signaling molecules, the stroma undergoes a process called decidualization where cells swell and stockpile resources, and specialized blood vessels called spiral arteries form to supply a potential embryo.10Human Reproduction. Morphological changes and protein secretion induced by progesterone in the endometrium during the luteal phase in preparation for nidation Progesterone also adjusts the local immune environment, regulating immune cell activity in the endometrium so that a genetically foreign embryo could implant without being attacked.11The Journal of Steroid Biochemistry and Molecular Biology. Oestrogen and progesterone regulation of inflammatory processes in the human endometrium
The luteal phase is remarkably consistent in length compared to the follicular phase, typically lasting about 12 to 14 days. If pregnancy occurs, the embryo produces a hormone (hCG) that rescues the corpus luteum and keeps progesterone flowing until the placenta can take over production. If no pregnancy occurs, the corpus luteum degenerates in a process called luteolysis. Progesterone and estrogen levels plummet, the endometrium loses its hormonal support, and the cycle resets with menstruation.12Human Reproduction. Different patterns of structural luteolysis in the human corpus luteum of menstruation
The Pulsing Signal That Drives Everything
One detail that gets lost in most summaries is that the brain doesn’t send a steady stream of hormonal signals to the ovaries. GnRH is released in pulses, and the speed of those pulses changes across the cycle. During the follicular phase, GnRH pulses come relatively fast, roughly once per hour, which favors LH release. During the luteal phase, progesterone from the corpus luteum slows the pulse frequency, which shifts the pituitary toward producing more FSH instead.13PubMed Central. GnRH pulses–the regulators of human reproduction Research measuring LH pulses every 10 minutes found that pulse frequency increases during the follicular phase and then drops during the luteal phase, though the change is more modest than older studies suggested.14PubMed. Pulsatile gonadotropin secretion during the human menstrual cycle: evidence for altered frequency of gonadotropin-releasing hormone secretion
This pulse-frequency shift is what allows the cycle to loop back on itself. As the corpus luteum dies and progesterone falls, the slowing effect on GnRH pulses lifts, FSH rises again, and a new batch of follicles is recruited. It’s an elegant feedback system: the ovary’s own output changes the brain’s signaling pattern, which in turn changes what the ovary does next.
Effects You Can Feel Beyond the Uterus
The hormonal swings of the cycle influence far more than the reproductive organs. One of the most well-known systemic effects is the shift in basal body temperature. After ovulation, progesterone causes a slight temperature increase, typically a few tenths of a degree. This shift has been used as a marker of ovulation for decades.15The Journal of Clinical Endocrinology & Metabolism. THE CAUSE OF PHYSIOLOGIC BASAL TEMPERATURE CHANGES IN WOMEN Fertility-awareness methods rely on this temperature uptick, combined with cervical mucus changes, to estimate the fertile window.
The immune system also shifts measurably across the cycle. A systematic review and meta-analysis found that certain markers of innate immunity, including white blood cell counts, monocytes, and neutrophils, are higher during the luteal phase compared to the follicular phase.16Acta Physiologica. The effects of menstrual cycle phases on immune function and inflammation at rest and after acute exercise: A systematic review and meta‐analysis Progesterone appears to be the driver, acting as a broad immunosuppressant that shifts the immune profile in a direction that would favor tolerating an embryo, sometimes described as a “window of vulnerability” after ovulation where certain immune defenses are dialed down.17Archives of Gynecology and Obstetrics. Impact of progesterone on the immune system in women: a systematic literature review
Period pain, or dysmenorrhea, also has a clear hormonal and biochemical explanation. When progesterone drops at the end of the luteal phase and the endometrium begins to break down, cells in the lining release prostaglandins. People who experience significant cramping tend to have higher prostaglandin production in their endometrial tissue, and these molecules cause the uterine muscle to contract intensely, restricting blood flow and producing the characteristic aching or cramping pain.18PubMed. Dysmenorrhoea and prostaglandins: pharmacological and therapeutic considerations This is why nonsteroidal anti-inflammatory drugs (like ibuprofen), which block prostaglandin synthesis, are the first-line treatment for menstrual cramps.
Mood changes in the luteal phase are another commonly reported experience. Premenstrual dysphoric disorder (PMDD), a severe form of premenstrual syndrome, appears to stem not from abnormal hormone levels but from an abnormal brain response to normal fluctuations. Research points to a breakdown in how a progesterone byproduct called allopregnanolone interacts with receptors in the brain that regulate anxiety and stress. In people with PMDD, the brain’s sensitivity to these normal hormonal changes appears to be dysregulated, leading to pronounced mood symptoms.19PubMed Central. Allopregnanolone in premenstrual dysphoric disorder (PMDD): Evidence for dysregulated sensitivity to GABA-A receptor modulating neuroactive steroids across the menstrual cycle
When the Cycle Doesn’t Follow the Textbook
The four-phase description above represents an idealized cycle, and real cycles deviate from it in a number of ways. The follicular phase is the primary source of variation in cycle length. While the luteal phase stays close to 14 days in most people, the follicular phase can stretch or compress substantially depending on how quickly a dominant follicle is selected.
Stress is one of the most common disruptors. Psychological stress raises cortisol, which can interfere with the hormonal cascade that triggers ovulation. In animal models, chronic stress leads to irregular cycles and failure to ovulate, driven by elevated cortisol and downstream effects on the ovary.20PubMed. Role of cortisol and superoxide dismutase in psychological stress induced anovulation In humans, the same pattern shows up: significant life stressors, illness, rapid weight changes, and intense exercise can all delay or prevent ovulation, which extends the follicular phase and makes the overall cycle longer or causes missed periods entirely.
Polycystic ovary syndrome (PCOS) disrupts the cycle at the follicular-selection step. In PCOS, a hormonal environment characterized by excess LH and insulin prevents any one follicle from becoming dominant. Many small follicles begin developing but stall at an intermediate size, and the resulting suppression of FSH blocks the maturation process.21Human Reproduction Update. Follicle dynamics and anovulation in polycystic ovary syndrome Without ovulation, no corpus luteum forms, progesterone stays low, and the endometrium either doesn’t shed on schedule or sheds irregularly.
Age is another factor. As people approach their late 30s and 40s, the follicular phase tends to shorten. Research on older reproductive-age women has found that they develop a dominant follicle faster than younger women, associated with a steeper early rise in FSH, which leads to shorter overall cycles.22The Journal of Clinical Endocrinology & Metabolism. Reproductive aging: accelerated ovarian follicular development associated with a monotropic follicle-stimulating hormone rise in normal older women This shortening is one of the earliest signs of the transition toward menopause, often appearing years before periods stop altogether.
How Hormonal Contraception Overrides the Cycle
Combined hormonal contraceptives (the pill, patch, or ring) work by hijacking the feedback system that makes the cycle run. They deliver synthetic estrogen and progestin in doses that suppress GnRH pulsing from the hypothalamus and prevent the FSH and LH surges needed for follicle development and ovulation.23PubMed Central. Hormonal contraception–what kind, when, and for whom? The progestin component is primarily responsible for blocking the LH surge, while the estrogen component helps suppress FSH and stabilizes the endometrium to prevent irregular bleeding.24Endocrine Reviews. Contraceptive Selection for the Endocrine Patient: What an Endocrinologist Should Know
The “period” you get on the pill during the placebo week is not a true menstrual period. It’s a withdrawal bleed caused by the sudden drop in synthetic hormones, mimicking the progesterone withdrawal that triggers real menstruation. Since no ovulation has occurred and the endometrial lining is thinner than in a natural cycle, the bleeding is typically lighter and less painful. Progestin-only methods (like hormonal IUDs, the implant, or the mini-pill) work somewhat differently. They thicken cervical mucus, thin the endometrium, and in some cases suppress ovulation, though not as reliably as combined methods. Many people on progestin-only methods experience lighter periods or stop bleeding altogether, which is safe and simply reflects the thin, quiet state of the endometrium.
Why Humans Menstruate at All
Most mammals don’t menstruate. Of more than 5,000 mammal species, overt menstruation occurs in only a handful of groups: some primates, certain bats, and the elephant shrew. This has puzzled biologists for a long time. Older theories proposed that menstruation served to flush out pathogens introduced by sperm, or that shedding the lining was more energy-efficient than maintaining it month after month.25PubMed. The significance and evolution of menstruation
More recent evolutionary thinking focuses on a process called spontaneous decidualization, the transformation of the endometrial lining that in humans happens automatically under progesterone’s influence each cycle, whether or not an embryo is present. In most mammals, decidualization only occurs when an embryo signals its arrival. The fact that humans decidualize spontaneously may be a defense mechanism: it gives the mother’s body a chance to “screen” embryos before investing resources in a pregnancy. The embryo must implant into already-transformed tissue, which limits how deeply it can invade. If no embryo implants, the decidualized tissue is shed, and that shedding is menstruation.26PubMed Central. The evolution of menstruation: a new model for genetic assimilation Under this model, menstruation isn’t the “point” of the cycle. It’s a byproduct of a protective strategy that evolved because of the unusually aggressive implantation behavior of human embryos compared to those of most other mammals.