Menstruation is the first phase of the menstrual cycle, commonly called the menstrual phase. It marks day one of a new cycle, the moment bleeding begins, and it sits within the broader follicular phase that continues until ovulation. While that placement sounds straightforward, the biology behind menstrual bleeding involves a cascade of hormonal shifts, immune activity, and tissue remodeling that makes the menstrual phase one of the most physiologically active windows of the entire cycle.
How the Menstrual Phase Fits Into the Full Cycle
The menstrual cycle is generally divided into two main halves, separated by ovulation. The first half is the follicular phase, which begins on the first day of menstrual bleeding and lasts until the ovary releases an egg. The second half is the luteal phase, which runs from ovulation until the next period starts. Menstruation occupies the opening days of the follicular phase, typically lasting between three and seven days, though this varies from person to person and even cycle to cycle.
Some descriptions break the cycle into three or four phases by splitting out the menstrual phase as its own segment and treating ovulation as a brief standalone event. Regardless of the labeling system, the key point is the same: day one of bleeding equals day one of the cycle. This convention matters because doctors, fertility trackers, and researchers all anchor their timelines to that first day of flow.
What Triggers Menstrual Bleeding
The trigger for menstruation is the withdrawal of progesterone. During the luteal phase, a temporary structure in the ovary called the corpus luteum pumps out progesterone to maintain the thickened uterine lining. If pregnancy doesn’t happen, the corpus luteum breaks down, progesterone levels drop sharply, and the endometrium loses its hormonal support.1PubMed. Physiology of the Endometrium and Regulation of Menstruation That decline in progesterone sets off an inflammatory process in the endometrium. The tissue swells, spiral blood vessels in the lining constrict, and enzymes that break down structural proteins ramp up dramatically.2PubMed Central. Menstrual physiology: implications for endometrial pathology and beyond
The result is the shedding of roughly the upper two-thirds of the endometrium, a layer known as the functional layer. Blood, tissue fragments, and fluid exit through the cervix and vagina as menstrual flow. The deeper basal layer stays intact and serves as the foundation from which the lining regenerates during the rest of the follicular phase. Estrogen levels also drop around this time but begin climbing again relatively quickly once new follicles start developing in the ovary. Progesterone, by contrast, stays very low until after ovulation, weeks later.3PubMed. Progesterone: a pivotal hormone at menstruation
Follicle Recruitment Starts Before Bleeding Stops
One of the less intuitive aspects of the menstrual phase is that the ovary isn’t just waiting around during those bleeding days. The rise in follicle-stimulating hormone (FSH) that kicks off the development of the next egg-bearing follicle actually begins before menstruation starts. Research analyzing hundreds of cycles found that this FSH rise typically begins about four days before the first day of bleeding, coinciding with the first drop in estrogen levels at the end of the luteal phase.4Oxford Academic (Human Reproduction). The onset of the initial rise in follicle-stimulating hormone during the human menstrual cycle So by the time you notice bleeding on day one, the ovary has already been quietly recruiting its next batch of follicles for several days.
This overlap explains why the menstrual phase is considered part of the follicular phase rather than a separate entity. The ovary and the uterus are on slightly different schedules: the uterus is finishing last cycle’s cleanup while the ovary is already starting the next cycle’s follicle selection. Both events are happening simultaneously, driven by the same hormonal shifts.
Why Cramps and Other Symptoms Happen During This Phase
Period cramps, medically called dysmenorrhea, are closely tied to what’s happening in the uterine lining during the menstrual phase. As the endometrium breaks down, it releases prostaglandins, particularly two types (PGF2α and PGE2) that cause the uterine muscle to contract. These contractions help expel the shedding tissue but can also temporarily reduce blood flow to the uterus, producing the characteristic cramping pain. Women with more painful periods tend to have higher concentrations of these prostaglandins in their endometrial tissue and menstrual fluid compared to those who experience little discomfort.5PubMed. Eicosanoids in primary dysmenorrhea, endometriosis and menstrual migraine
The prostaglandin connection also explains why over-the-counter anti-inflammatory painkillers like ibuprofen work well for period cramps. These drugs block prostaglandin production. The same prostaglandins can spill into the bloodstream and affect other organs, which is why some people experience nausea, diarrhea, headaches, or general achiness during the first day or two of their period. These aren’t separate problems; they’re downstream effects of the same inflammatory molecules driving the shedding process.
Anovulatory Bleeding Is Not the Same as Menstruation
Not every episode of uterine bleeding is true menstruation. Menstruation, strictly defined, is the bleeding that follows an ovulatory cycle where progesterone rose and then withdrew. But cycles don’t always include ovulation, especially during adolescence, perimenopause, or in people with conditions like polycystic ovary syndrome. When ovulation doesn’t happen, there’s no corpus luteum and therefore no progesterone surge. The endometrium still builds up under estrogen’s influence, but without progesterone’s organizing effects, it eventually breaks down in a disorganized way.
This anovulatory bleeding can look like a period, but the underlying biology is different. Research has shown that the biochemical processes in normal menstruation are well-ordered: progesterone withdrawal leads to controlled changes in clotting factors, enzyme activity, and inflammatory signals that produce a predictable bleed. Anovulatory bleeding, by contrast, reflects an endometrium that never received progesterone’s stabilizing influence, leaving it structurally fragile and prone to unpredictable, sometimes heavy or prolonged bleeding.6PubMed Central. Mechanisms of normal and abnormal endometrial bleeding This distinction matters clinically because irregular heavy bleeding in someone who isn’t ovulating points to a different set of causes and treatments than heavy but regular periods.
How Cycle Length Variability Affects the Menstrual Phase
When people talk about a “28-day cycle,” they’re describing an average that obscures enormous variation. Cycle length differs between individuals, and it also shifts within the same person from month to month. A prospective study tracking over 670 ovulatory cycles in healthy women found that most of the variation in total cycle length comes from the follicular phase (the half that includes menstruation), not the luteal phase. Within-woman variability for follicular phase length had a median of about five days, meaning the same person’s follicular phase could easily be several days shorter or longer from one cycle to the next.7PubMed Central. Prospective 1-year assessment of within-woman variability of follicular and luteal phase lengths in healthy women prescreened to have normal menstrual cycle and luteal phase lengths
The luteal phase, by comparison, is relatively stable, with within-woman variability around three days. This means that if your cycle is “late” by a few days, it’s almost always because your follicular phase (and potentially the timing of ovulation) shifted, not because your luteal phase stretched. The menstrual phase itself, as the opening portion of the follicular phase, tends to be fairly consistent in duration for a given individual, but the total time from the start of bleeding to ovulation is the flexible part.
Adolescent Cycles and Early Irregularity
The menstrual phase behaves differently in the first few years after menarche (the first period). Adolescents are far more likely to have anovulatory cycles, meaning the “menstrual” bleeding they experience often isn’t true progesterone-withdrawal menstruation at all. A study of U.S. adolescents found that those less than a year past menarche had about five times the odds of having short cycles and about two-and-a-half times the odds of having highly variable cycles compared to those who were six or more years past menarche.8PubMed. Menstrual Cycle Characteristics of U. S. Adolescents According to Gynecologic Age and Age at Menarche The same study found that the odds of experiencing heavy flow increased with time since menarche, which tracks with the transition toward more consistently ovulatory cycles where progesterone-driven endometrial buildup produces a heavier but more predictable bleed.
This early irregularity is normal and reflects a maturing reproductive axis that hasn’t yet settled into a reliable pattern of ovulation. It can take three to five years after menarche for cycles to become consistently ovulatory. During that maturation window, cycle lengths, flow duration, and symptoms can be quite unpredictable.
Body Temperature and Metabolism Around Menstruation
One of the more practical consequences of the hormonal shifts around menstruation involves body temperature. Core body temperature runs about 0.3°C to 0.7°C higher during the luteal phase (after ovulation) compared to the follicular phase, driven primarily by progesterone’s effect on the brain’s temperature-regulating center.9PubMed Central. Temperature regulation in women: Effects of the menstrual cycle When progesterone drops and menstruation begins, body temperature falls back to its lower baseline. This is the principle behind basal body temperature tracking: a sustained temperature rise confirms that ovulation has occurred, and the subsequent drop signals the approach of menstruation.
The temperature shift is most noticeable when measured first thing in the morning before getting out of bed. Age affects the pattern, too. Research using data from large populations found that the temperature difference between follicular and luteal phases gradually increased through the late twenties, stabilized, and then started to diminish after the early forties as cycles become less consistently ovulatory.10PubMed Central. Age-Dependent and Seasonal Changes in Menstrual Cycle Length and Body Temperature Based on Big Data
Metabolic rate follows a similar rhythm. Resting metabolic rate tends to dip around the time of menstruation and reaches its lowest point about a week before ovulation. It then climbs through the luteal phase, peaking shortly before the next period.11PubMed. Menstrual cycle and basal metabolic rate in women A meta-analysis pooling data from dozens of studies confirmed a small but consistent increase in resting metabolic rate during the luteal phase compared to the follicular phase.12PLoS ONE. Effect of menstrual cycle on resting metabolism: A systematic review and meta-analysis This means the body burns slightly fewer calories at rest during and just after menstruation than it does in the second half of the cycle. The difference is modest, but it’s real, and it partially explains why appetite and energy levels can fluctuate across the month.
The Immune Landscape During Menstrual Shedding and Repair
Menstruation is sometimes described as a “controlled wound.” The shedding of the functional endometrial layer leaves a raw surface that must be rebuilt quickly, and the immune system plays a central role in both the breakdown and the repair. Recent research using detailed immune-cell profiling in animal models of menstruation found that the types of immune cells present in the uterus shift dramatically between the tissue breakdown, repair, and remodeling stages. Monocytes, macrophages, and neutrophils appear to be particularly important during the repair window, clearing debris and coordinating the regeneration of the lining.13bioRxiv. Multimodal Profiling of Repair-associated Immune Dynamics in a Mouse Model of Menstruation
This immune activation is part of why the menstrual phase has broader effects on the body. The inflammatory signals aren’t perfectly contained within the uterus. Some people notice increased susceptibility to cold sores, mild joint achiness, or general fatigue during the first days of their period, all of which may relate to the systemic ripple effects of localized inflammation in the endometrium.
What Happens to the Vaginal Microbiome During Menstruation
The vaginal environment shifts noticeably during menstrual bleeding. Menstrual fluid raises the vaginal pH (making it less acidic) and introduces blood and tissue into an environment that normally has a stable bacterial community. Research tracking daily vaginal samples found that microbial diversity increases during menses while protective Lactobacillus bacteria decrease.14PubMed Central. Daily Vaginal Microbiota Fluctuations Associated with Natural Hormonal Cycle, Contraceptives, Diet, and Exercise The Lactobacillus population typically bounces back after bleeding ends and estrogen levels climb again, restoring the acidic environment that keeps other organisms in check.
This temporary disruption is one reason some people are more prone to yeast infections or bacterial vaginosis around the time of their period. It’s not that menstruation “causes” these infections, but the shift in pH and bacterial balance creates a window of vulnerability. Practices that further disrupt vaginal pH during menstruation, like douching, can extend that window.
Withdrawal Bleeds on Hormonal Contraception
If you use combined hormonal contraceptives (the pill, the patch, or the ring), the bleeding you get during the hormone-free interval is not true menstruation. It’s a withdrawal bleed triggered by the drop in synthetic hormones when you take the placebo pills or remove the patch or ring. The underlying mechanism is superficially similar to menstruation in that hormone withdrawal causes the lining to shed, but the endometrium on hormonal contraception is typically much thinner because ovulation has been suppressed and progestin exposure is continuous throughout the active pills. The result is usually lighter, shorter, and less painful bleeding than a natural period.
There is no medical need for this withdrawal bleed. It was built into the original pill design in the 1960s partly to make the contraceptive feel more “natural” and partly to win approval from religious authorities by mimicking the menstrual cycle. Research into attitudes toward menstruation on contraception has found that many young women still value having a regular bleed as a sign of non-pregnancy and a reassuring connection to a natural cycle, even when they know the withdrawal bleed has no physiological purpose.15BMJ Sexual & Reproductive Health. Hormonal contraception and regulation of menstruation: a study of young women’s attitudes towards ‘having a period’ Continuous or extended-cycle pill regimens that skip the placebo week are safe and increasingly common for people who prefer to avoid monthly bleeding altogether.
Why Humans Menstruate at All
Most mammals don’t menstruate. Among the roughly 5,000 species of mammals, overt menstruation occurs in only a handful of lineages: higher primates (including humans, apes, and Old World monkeys), some bats, and the elephant shrew. This rarity has puzzled biologists for over a century. Early theories suggested menstruation served to flush pathogens introduced by sperm, or that it was more energy-efficient to shed and rebuild the lining each month than to maintain it continuously. Neither theory held up well under scrutiny.16PubMed. The significance and evolution of menstruation
More recent evolutionary models focus on a process called spontaneous decidualization. In most mammals, the uterine lining only transforms into its receptive, pregnancy-ready state in response to signals from an embryo. In menstruating species, the lining transforms spontaneously under the influence of progesterone, without needing an embryo present. This preemptive transformation may have evolved as a way for the mother’s body to “screen” embryos before allowing deep implantation, providing some protection against genetically abnormal pregnancies or excessively invasive placental tissue.17PubMed Central. The evolution of menstruation: a new model for genetic assimilation Menstruation, in this view, is the consequence of that spontaneous preparation: when no embryo implants, the transformed lining has to be discarded and rebuilt. The bleeding isn’t the point. It’s the side effect of a protective strategy that gives the mother more control over which pregnancies proceed.