Most of us learned the menstrual cycle as a tidy 28-day circle: bleed, build, release, wait, repeat. The real thing is less tidy and more interesting. Your cycle is a monthly conversation between the brain, the ovaries and the uterus, carried by a handful of hormones that rise and fall in a predictable order, even if the exact timing shifts from one month to the next.
Understanding that order is useful. It explains why your temperature changes, why a late period is not always a sign of anything, and why so much of the advice sold as "cycle syncing" rests on thinner evidence than its confidence suggests.
This article covers the four phases in the order they happen, with the hormones and the numbers behind each one. If you want a closer look at the second half of the cycle, where bloating, cravings and mood changes tend to cluster, read our companion guide to the luteal phase.
How the Cycle Is Counted
A menstrual cycle is counted from the first day of one period to the first day of the next. Day 1 is the first day of full bleeding, not spotting. That convention matters, because almost every number in this article (and in your doctor's notes) is measured from it.
The 28-day figure is an average, not a target. In 2019 a team including researchers from University College London and the Karolinska Institute analyzed 612,613 ovulatory cycles recorded by 124,648 app users, published in npj Digital Medicine. The mean cycle length was 29.3 days, and only 13 percent of cycles were exactly 28 days long. The study was funded by the app company and its users are not a random sample of the population, so read the exact figures with some care, but the message is consistent with clinical guidance: a wide range of lengths is normal.
The same study also found that cycle length fell by about 0.18 days per year of age between 25 and 45, which adds up to roughly three days over those two decades. The Mayo Clinic describes the same arc in plain terms: cycles are often longer in the first years after periods begin, tend to become shorter and more regular with age, and may become irregular again in the approach to menopause.
One more framing worth knowing. The American College of Obstetricians and Gynecologists regards the menstrual cycle as a "vital sign," read in the same way as pulse or blood pressure, because a cycle that drifts well outside the normal range can be an early signal of something worth checking. That position is set out in ACOG Committee Opinion No. 651.
Phase 1: The Menstrual Phase
When: roughly days 1 to 5. Hormones: estrogen and progesterone at their lowest.
The cycle begins with the period, which is the body shedding the lining it built up during the previous cycle. The trigger is hormonal: when no pregnancy occurs, the structure that was producing progesterone breaks down, progesterone and estrogen fall, and the uterine lining can no longer be maintained. It separates and leaves the body as menstrual blood and tissue.
According to the NHS, bleeding lasts 2 to 7 days, usually about 5, with the first two days typically the heaviest. Total blood loss over a period is around 20 to 90 ml, or about one to five tablespoons, although some women lose more.
Strictly speaking, menstruation overlaps with the start of the follicular phase, because the ovaries begin the next cycle's work while the uterus is still clearing the last one. It is listed on its own because it is the part you can see.
Phase 2: The Follicular Phase
When: day 1 until ovulation, most often about 10 to 20 days. Hormones: follicle-stimulating hormone (FSH) early, then rising estrogen.
The follicular phase takes its name from the follicles: tiny fluid-filled sacs in the ovaries, each holding an immature egg. As the period begins, the pituitary gland at the base of the brain releases FSH, which prompts a group of follicles to start growing. Over the next week or so, one follicle becomes dominant and the rest fade.
As the dominant follicle grows, it produces increasing amounts of estrogen. Estrogen does two things at once. In the uterus it rebuilds the lining that was just shed, thickening it into a receptive bed for a possible pregnancy. In the brain it gradually changes the signal going back to the pituitary, setting up the hormonal surge that will release the egg.
This is the phase that varies most. In the 600,000-cycle dataset, the mean follicular phase was 16.9 days, but the reported range ran from 10 to 30 days. When a cycle is unusually long or short, the difference almost always sits here, in how long the body takes to bring one follicle to maturity. Stress, illness, travel and weight change can all stretch or shorten it. The second half of the cycle, by contrast, is comparatively fixed.
Phase 3: Ovulation
When: a single day, roughly mid-cycle. Hormones: a sharp surge of luteinizing hormone (LH), triggered by peak estrogen.
Once estrogen stays high for long enough, the brain flips its response. Instead of holding LH back, the pituitary releases a sudden surge of it. The Office on Women's Health describes LH beginning to rise about 36 hours before ovulation and peaking about 12 hours before the egg is released. This surge is what over-the-counter ovulation tests detect.
The dominant follicle then ruptures and releases its egg into the fallopian tube. The egg survives for only 12 to 24 hours. Sperm can live for 3 to 5 days inside the body, which is why the fertile window opens before ovulation rather than on the day itself.
How predictable is that window? Less than the calendar suggests. A prospective study of 221 women and 696 cycles published in the BMJ in 2000 used urinary hormone markers to pin down ovulation day. The six-day fertile window (the five days before ovulation plus the day itself) fell entirely within the "textbook" days 10 to 17 for only about 30 percent of women. Every day from day 6 to day 21 carried at least a 10 percent chance of being inside someone's fertile window, and the probability peaked on days 12 and 13. The authors' conclusion: even women with regular cycles cannot reliably predict their fertile days from the calendar alone.
Phase 4: The Luteal Phase
When: ovulation until the next period, usually 12 to 14 days. Hormones: progesterone dominant, with a secondary rise in estrogen.
After releasing its egg, the empty follicle does not simply disappear. It reorganizes into a temporary gland called the corpus luteum (Latin for "yellow body"), which produces progesterone. Progesterone's job is to hold the uterine lining steady and make it secretory and nourishing, ready to receive a fertilized egg.
Progesterone also has effects well beyond the uterus. It is mildly thermogenic: resting body temperature runs about 0.3 to 0.4 °C (roughly 0.5 to 0.7 °F) higher in the luteal phase than in the follicular phase, as summarized in a 2010 review in the International Journal of Endocrinology. Sleep architecture shifts slightly, with less REM sleep and more spindle activity in the luteal phase, and self-reported sleep quality tends to be worse in the premenstrual days, particularly for women who also have premenstrual symptoms or painful periods, according to a 2018 review in Sleep Medicine Clinics.
If no pregnancy begins, the corpus luteum has a fixed lifespan. It degrades after roughly two weeks, progesterone and estrogen fall away, and the drop triggers the next period. This is why the luteal phase is the more constant half of the cycle: in the large app dataset its mean length was 12.4 days with a standard deviation of only 2.4 days. The NHS puts it another way: ovulation usually happens about 12 to 16 days before the next period starts, however long the cycle is overall.
The premenstrual symptoms many women notice (bloating, breast tenderness, food cravings, lower mood) cluster in the last several days of this phase, as hormone levels fall. We go into the mechanisms in detail in The Luteal Phase Explained, and into the clinical picture in What Is PMS?.
The Four Phases at a Glance
| Phase | Typical timing | Dominant hormone | What is happening |
|---|---|---|---|
| Menstrual | Days 1 to 5 (2 to 7 days of bleeding) | Estrogen and progesterone low | The uterine lining from the last cycle is shed |
| Follicular | Day 1 to ovulation; about 10 to 20 days, highly variable | FSH, then rising estrogen | Follicles grow, one becomes dominant, the lining rebuilds |
| Ovulation | One day, roughly mid-cycle | LH surge | The egg is released and survives 12 to 24 hours |
| Luteal | Ovulation to next period; about 12 to 14 days, fairly constant | Progesterone, with some estrogen | The corpus luteum sustains the lining, then breaks down |
Timings are drawn from the NHS, the Office on Women's Health and the 2019 npj Digital Medicine analysis. Your own cycle may sit outside these windows and still be healthy.
What Counts as a Normal Cycle Length
Guidance differs slightly by source, but the ranges overlap closely:
- The Office on Women's Health considers periods regular if they come every 24 to 38 days, and notes that the time from ovulation to the next period can range from 7 to 19 days.
- The Mayo Clinic uses the same 24-to-38-day window and allows bleeding of up to 8 days.
- The NHS describes cycles of 21 to 35 days as common, with bleeding of 2 to 7 days.
Within those ranges, "normal" is really "normal for you." A woman whose cycle is reliably 25 days is no less typical than one whose cycle is reliably 33. Cycle-to-cycle variation of a few days is also ordinary. What clinicians pay more attention to is change: a cycle that was regular and becomes irregular, bleeding that becomes much heavier, or a gap of three months or more without a period when pregnancy and breastfeeding have been ruled out.
Hormonal contraception changes the picture entirely. The bleeding that happens on the pill, patch or ring is a withdrawal bleed scheduled by the hormone-free days, not a true period that follows ovulation, and hormonal IUDs can make bleeding lighter or absent. The four-phase description in this article applies to natural cycles.
The Fourth Trimester makes teas from dried culinary and herbal ingredients, drawn from recipes traditionally prepared in China around the menstrual cycle. That is the tradition behind our Premenstrual Tea.
Explore the Menstrual Cycle rangeCycle Syncing: What the Evidence Actually Shows
"Cycle syncing" is the idea that you should plan your workouts, meals, work and social life around the four phases: lift heavy in the follicular phase, go gentle in the luteal phase, eat certain foods at certain times. It is a popular framework on social media, and parts of it are intuitive. The hormonal changes described above are real. The question is whether they are large enough to justify reorganizing your month around them.
For exercise, the best available answer comes from two large reviews.
The first, a systematic review and meta-analysis of 78 studies published in Sports Medicine in 2020, compared exercise performance across cycle phases in women with regular cycles. Its conclusion was that performance "might be trivially reduced" during the early follicular phase (the period days) compared with the rest of the cycle, with large variation between studies. The authors classified the overall quality of the evidence as low, and recommended an individual approach rather than general rules.
The second, an umbrella review of existing meta-analyses and systematic reviews published in Frontiers in Sports and Active Living in 2023, looked specifically at strength and muscle gain. The authors judged the underlying evidence to be low quality and inconsistent, said it was "premature to conclude" that short-term hormone fluctuations meaningfully affect performance, and stated plainly that resistance-training programs built around cycle phase are not evidence-based. A recurring problem they identified is that many studies never verified which phase participants were actually in, relying instead on counting days from the last period, which, as the BMJ fertile-window study above shows, is unreliable.
What does this mean in practice?
- If you feel stronger at one point in your cycle and sluggish at another, that is worth noticing and worth working with. Individual variation is real, and the reviews say as much.
- There is no good evidence that a general schedule (heavy lifting in week two, yoga in week four) produces better results than training when you feel able to.
- Claims about syncing specific foods or herbs to specific phases rest largely on tradition and plausibility rather than trials. That does not make them wrong, but it is worth knowing which is which. The Chinese tradition is the oldest systematic version of this idea, tiao jing, and is candid about being a tradition; our article on Traditional Chinese Medicine and the cycle explains it. Our guide to foods and teas traditionally taken around your period tries to keep the two clearly separated.
The most defensible version of cycle syncing is the simplest one: track your cycle, notice your own patterns, and adjust for them. That is also, not coincidentally, what the clinical guidance says to do.
When to Talk to a Clinician
Variation is normal, but some patterns deserve a conversation with a doctor, nurse or midwife. The Mayo Clinic and the Office on Women's Health both suggest getting in touch if:
- Your cycles are shorter than 24 days or longer than 38 days.
- Your periods stop for more than 90 days and you are not pregnant or breastfeeding.
- Your periods become irregular after having been regular.
- You bleed for more than 8 days, soak through a pad or tampon every hour or two, or pass clots larger than a quarter.
- You bleed between periods.
- You have severe pain with your period.
- A girl has not started her period by age 15, or cycles remain long or irregular more than three years after the first period.
None of these necessarily means something is wrong, but they are the signals clinicians use, and bringing a few months of tracked dates to the appointment makes the conversation far more useful.
Frequently Asked Questions
What are the four phases of the menstrual cycle?
The four phases are the menstrual phase (the period, usually days 1 to 5), the follicular phase (from day 1 until ovulation, when follicles grow and estrogen rises), ovulation (the release of the egg, triggered by an LH surge around mid-cycle) and the luteal phase (ovulation until the next period, when progesterone from the corpus luteum dominates). Menstruation technically overlaps with the start of the follicular phase.
How long is a normal menstrual cycle?
Cycles of 24 to 38 days are considered regular by the Office on Women's Health and the Mayo Clinic, and the NHS describes 21 to 35 days as common. A study of more than 600,000 cycles found a mean length of 29.3 days, with only 13 percent of cycles lasting exactly 28 days. Most of the variation comes from the follicular phase; the luteal phase is usually about 12 to 14 days.
Does cycle syncing work?
The evidence is thin. A 2020 meta-analysis of 78 studies found exercise performance may be only trivially lower during the period, with low-quality evidence overall, and a 2023 umbrella review concluded that resistance training programs built around cycle phase are not evidence-based. Tracking your own cycle and adjusting to how you feel is reasonable; a fixed schedule for everyone is not supported by the research.
References
Bull, J.R., Rowland, S.P., Scherwitzl, E.B., Scherwitzl, R., Gemzell Danielsson, K. & Harper, J. (2019). Real-world menstrual cycle characteristics of more than 600,000 menstrual cycles. npj Digital Medicine, 2, 83. PMC
American College of Obstetricians and Gynecologists (2015). ACOG Committee Opinion No. 651: Menstruation in Girls and Adolescents: Using the Menstrual Cycle as a Vital Sign. Obstetrics & Gynecology, 126(6), e143 to e146. PubMed
Office on Women's Health, U.S. Department of Health and Human Services. Your menstrual cycle. womenshealth.gov
National Health Service. Periods: Overview. NHS
Mayo Clinic Staff (2026). Menstrual cycle: What's typical, what's not. Mayo Clinic
Wilcox, A.J., Dunson, D. & Baird, D.D. (2000). The timing of the "fertile window" in the menstrual cycle: day specific estimates from a prospective study. BMJ, 321(7271), 1259 to 1262. PMC
McNulty, K.L., Elliott-Sale, K.J., Dolan, E., Swinton, P.A., Ansdell, P., Goodall, S., Thomas, K. & Hicks, K.M. (2020). The Effects of Menstrual Cycle Phase on Exercise Performance in Eumenorrheic Women: A Systematic Review and Meta-Analysis. Sports Medicine, 50(10), 1813 to 1827. PMC
Colenso-Semple, L.M., D'Souza, A.C., Elliott-Sale, K.J. & Phillips, S.M. (2023). Current evidence shows no influence of women's menstrual cycle phase on acute strength performance or adaptations to resistance exercise training. Frontiers in Sports and Active Living, 5, 1054542. PMC
Shechter, A. & Boivin, D.B. (2010). Sleep, Hormones, and Circadian Rhythms throughout the Menstrual Cycle in Healthy Women and Women with Premenstrual Dysphoric Disorder. International Journal of Endocrinology, 2010, 259345. PMC
Baker, F.C. & Lee, K.A. (2018). Menstrual Cycle Effects on Sleep. Sleep Medicine Clinics, 13(3), 283 to 294. PubMed