Menstrual Cycle: Hormones & Phases
Quick answer: The menstrual cycle is a roughly 28-day sequence in which hormones from the hypothalamus, pituitary and ovaries prepare the uterus for pregnancy. FSH grows the follicle and estrogen rebuilds the lining; peak estrogen triggers the LH surge and ovulation around day 14; the corpus luteum then secretes progesterone until, without fertilization, hormone levels crash and menstruation begins.
Key Facts: Menstrual Cycle
- The human menstrual cycle averages 28 days and is controlled by four hormones: FSH and LH (anterior pituitary) and estrogen and progesterone (ovaries).
- Ovulation occurs around day 14, triggered by the LH surge — a burst of LH caused by positive feedback from peak estrogen.
- The cycle has four phases: menstrual (days 1–5), follicular/proliferative (days 1–13), ovulatory (day ~14) and luteal/secretory (days 15–28).
- The corpus luteum secretes progesterone to maintain the endometrium; without fertilization it degenerates into the corpus albicans and menstruation begins.
- Menarche (first menstruation, around puberty) opens reproductive life; menopause (around 45–50 years) closes it.
- Each egg survives only about 24 hours after ovulation, while sperm survive up to about 5 days — so the fertile window spans roughly days 10–15.
If you memorize just one number from this whole chapter, make it 14. Day 14 is ovulation day in a standard 28-day cycle, and almost every exam question about the menstrual cycle circles back to it. The cycle is really two synchronized stories running at once: the ovarian cycle (what the follicle is doing) and the uterine cycle (what the endometrium is doing), both choreographed by hormones. Once you see how the four hormones hand the baton to each other, the whole sequence becomes one logical chain instead of four disconnected facts.
Day 1 is always the first day of menstrual bleeding. That is the anchor for every day number in this article. Real cycles vary (21–35 days is considered normal), and when a cycle is longer or shorter, it is almost always the follicular phase that stretches or shrinks — the luteal phase is remarkably constant at about 14 days in nearly all women, because the corpus luteum has a fixed lifespan.
The Hormones of the Cycle
Five hormones run the show, arranged in a strict chain of command from brain to ovary:
| Hormone | Source | Role in the cycle |
|---|---|---|
| GnRH (gonadotropin-releasing hormone) | Hypothalamus | Stimulates the anterior pituitary to release FSH and LH |
| FSH (follicle-stimulating hormone) | Anterior pituitary | Stimulates growth of ovarian follicles and estrogen secretion |
| LH (luteinizing hormone) | Anterior pituitary | LH surge causes ovulation; maintains the corpus luteum |
| Estrogen | Growing follicles (ovaries) | Regenerates the endometrium; at peak, triggers the LH surge (positive feedback) |
| Progesterone | Corpus luteum (ovaries) | Thickens and maintains the endometrium; inhibits FSH/LH (negative feedback) |
The logic is simple enough to carry into the exam hall: the brain starts it (GnRH), the pituitary drives the ovary (FSH, LH), and the ovary talks back to the uterus and the brain (estrogen, progesterone). Estrogen dominates the first half of the cycle and progesterone dominates the second. Every phase below is just these two ovarian hormones taking turns.
Phase 1: Menstrual Phase (Days 1–5)
Because no fertilization occurred in the previous cycle, the corpus luteum degenerates, so estrogen and progesterone levels crash. Without these hormones the endometrium (uterine lining) cannot be maintained: it breaks down and is shed along with blood through the vagina. This is menstruation, lasting about 4–5 days. The functional layer of the endometrium is lost; only the thin basal layer remains, and it is from this basal layer that the lining will regrow in the next phase.
There is a silver lining in the hormone crash: with estrogen and progesterone gone, their inhibitory grip on the pituitary is lifted, so FSH begins to rise and a brand new cycle starts. Menstruation is therefore not just the end of the old cycle — it is the hormonal starting gun of the new one.
Phase 2: Follicular (Proliferative) Phase (Days 1–13)
Under rising FSH, several primary follicles in the ovary begin to develop, but usually only one dominant follicle matures all the way into a fluid-filled Graafian follicle; the rest wither through atresia. The growing follicle secretes increasing amounts of estrogen, which does two jobs at once:
- it stimulates regeneration and thickening of the endometrium — this is why the phase is also called the proliferative phase;
- it holds FSH and LH down through negative feedback — but only until estrogen climbs very high.
Late in this phase something dramatic happens: the very high estrogen level flips to positive feedback on the hypothalamus and pituitary, forcing out a massive burst of LH. That burst is the LH surge, the single most exam-worthy event in the entire cycle.
Phase 3: Ovulatory Phase (Around Day 14)
The LH surge ruptures the mature Graafian follicle and releases the secondary oocyte into the fallopian tube — this is ovulation, landing around day 13–15 of a 28-day cycle. Note what is actually released: not a mature ovum but a secondary oocyte arrested in metaphase II; it completes meiosis II only if a sperm fertilizes it. A smaller FSH surge rides along with the LH surge and helps the follicle rupture.
The released egg survives only about 24 hours. Sperm, by contrast, can survive up to about 5 days in the female reproductive tract. That is why the fertile window is wider than a single day: intercourse in the several days before ovulation can still lead to fertilization. Compare this with the male side, where spermatogenesis produces millions of sperm continuously — the female system releases one egg per cycle, on a strict timetable.
Phase 4: Luteal (Secretory) Phase (Days 15–28)
The ruptured follicle does not just sit there. Its remains rebuild into the corpus luteum ("yellow body"), which secretes large amounts of progesterone (plus some estrogen). Progesterone:
- further thickens the endometrium and turns its glands secretory — producing glycogen-rich secretions that would nourish an early embryo;
- inhibits FSH and LH by negative feedback, so no new follicles start developing;
- raises the basal body temperature slightly (about 0.5 °C), a measurable sign used in fertility tracking.
If fertilization does not occur: the corpus luteum has a built-in expiry date of about 14 days. It degenerates into a white scar, the corpus albicans; progesterone and estrogen fall sharply; the endometrium collapses; and the whole story restarts with menstruation. If fertilization occurs: the implanting embryo releases hCG (human chorionic gonadotropin), which rescues the corpus luteum. Progesterone stays high, the lining is kept, and the cycle pauses for pregnancy — this is also the hormone detected by pregnancy tests.
Menstrual Cycle Hormone Chart: Regulation at a Glance
This one-page chart puts hormonal regulation of the menstrual cycle on a single timeline. Match each day range to its dominant hormone and uterine event — examiners love asking "which hormone peaks on day 14?" and "which phase has the highest progesterone?":
| Phase | Days | Estrogen | Progesterone | FSH / LH | Uterine & ovarian event |
|---|---|---|---|---|---|
| Menstrual | 1–5 | Low (crashing) | Low (crashing) | FSH begins to rise | Endometrium shed; bleeding |
| Follicular / proliferative | 1–13 | Rising to peak | Low | FSH high; LH surge at the end | Follicle matures; lining regenerates |
| Ovulatory | ~14 | Peak (positive feedback) | Low | LH surge + smaller FSH surge | Graafian follicle ruptures; egg released |
| Luteal / secretory | 15–28 | Moderate | High (dominant) | Both inhibited (negative feedback) | Corpus luteum maintains thick, secretory lining |
Feedback Loops: The Key to Hormonal Regulation
Hormonal regulation works through feedback loops between the ovary, pituitary and hypothalamus. Two operate in the cycle, and confusing them is one of the most common exam mistakes:
- Negative feedback (most of the cycle): rising estrogen and progesterone inhibit GnRH, FSH and LH release.
- Early follicular phase: low estrogen weakly inhibits FSH, so FSH can still rise and grow the follicle.
- Mid follicular phase: rising estrogen strengthens the inhibition, preventing extra follicles from being recruited.
- Luteal phase: high progesterone (with estrogen) strongly suppresses FSH and LH, so no new follicle develops while the lining is maintained.
- Positive feedback (just before ovulation): very high estrogen stimulates GnRH and LH release instead of inhibiting it.
- Sustained high estrogen from the mature Graafian follicle flips the hypothalamus–pituitary response.
- The result is the LH surge — a sudden flood of LH that triggers ovulation within about a day.
- This is the textbook's favourite example of positive feedback in human physiology, and examiners ask for it by name.
What Happens After Fertilization
If a sperm meets the secondary oocyte in the fallopian tube, fertilization completes meiosis II and forms the zygote, which immediately begins cleavage — rapid mitotic divisions as it travels toward the uterus. Around day 20–24 of the cycle the blastocyst implants into the thickened endometrium, and its outer cells begin secreting hCG. The hCG rescues the corpus luteum, progesterone stays high, and the uterine lining is maintained instead of being shed. Over the following weeks the developing extraembryonic membranes (amnion, chorion and others) form around the embryo, and the placenta eventually takes over progesterone production from the corpus luteum.
The Menstrual Cycle in Other Mammals
True menstruation — the visible shedding of the endometrium — is surprisingly rare in the animal kingdom. Most mammals reabsorb the endometrium instead (the estrous cycle):
- Primates (humans, apes, Old World monkeys), some bats and the elephant shrew menstruate visibly.
- Most other placental mammals (dogs, cows, rodents) have estrous cycles: the endometrium is reabsorbed if there is no pregnancy, and females are sexually receptive only during "heat."
- Marsupials have very short pregnancies and no true menstrual cycle.
- Monotremes (platypus, echidnas) lay eggs and have no cycle of this kind at all.
For how these reproductive strategies differ across mammals — egg-laying, pouched and placental — see the comparison of monotremes vs marsupials vs placentals.
Menarche, Menopause and Cycle Variations
- Menarche is the first menstrual bleeding, occurring around puberty (typically 12–13 years). It marks the start of reproductive life but does not mean full fertility — early cycles are often irregular and anovulatory.
- Menopause is the permanent stoppage of menstruation around 45–50 years, when the ovaries run out of follicles. Estrogen falls permanently, ending the cycles.
- Cycle length: 21–35 days is normal in adults. Variation comes almost entirely from the follicular phase; the luteal phase stays near 14 days because the corpus luteum has a fixed lifespan.
- Amenorrhea (absent periods) can signal pregnancy, extreme weight loss, excessive exercise or hormonal disorders — a common one-line exam definition.
Draw the cycle as one timeline table with four columns: phase, days, dominant hormone(s), and ovarian/uterine events. Examiners award marks for: ovulation on day 14, the LH surge causing ovulation, estrogen driving the follicular phase, progesterone driving the luteal phase, and the corpus luteum → corpus albicans sequence when fertilization fails. Never confuse the follicular (proliferative) phase (estrogen's show) with the luteal (secretory) phase (progesterone's show). If asked for positive feedback, write "estrogen triggering the LH surge" — that single line is almost always worth a full mark.
Frequently Asked Questions
Which hormone actually causes ovulation?
The LH surge. Around day 14, the anterior pituitary releases a sudden flood of luteinizing hormone, and that flood ruptures the Graafian follicle to release the secondary oocyte. If a question asks for one hormone behind ovulation, LH is the answer.
What is the corpus luteum, and what does it secrete?
The corpus luteum ("yellow body") is the transformed remains of the ruptured Graafian follicle after ovulation. It secretes large amounts of progesterone (plus some estrogen), which thickens the endometrium, makes its glands secretory, and inhibits FSH/LH by negative feedback. Without fertilization it degenerates into the corpus albicans.
Why does menstrual bleeding happen?
Because the body prepared for a pregnancy that never came. With no fertilization, the corpus luteum withers, so estrogen and progesterone fall sharply. The endometrial lining cannot survive without them, so it breaks down and leaves as menstrual bleeding lasting about 4–5 days.
What are the four phases of the menstrual cycle?
(1) Menstrual phase (days 1–5): the lining is shed. (2) Follicular/proliferative phase (days 1–13): FSH grows the follicle and estrogen rebuilds the lining. (3) Ovulatory phase (around day 14): the LH surge releases the egg. (4) Luteal/secretory phase (days 15–28): the corpus luteum secretes progesterone to maintain the lining.
What is the difference between menarche and menopause?
Menarche is the first menstrual bleeding at puberty, marking the start of reproductive life. Menopause is the permanent stoppage of menstruation around age 45–50, when the ovarian follicles are exhausted.
How do negative and positive feedback differ in the cycle?
Negative feedback operates through most of the cycle: estrogen and progesterone inhibit GnRH, FSH and LH, keeping levels balanced and preventing extra follicles. Positive feedback happens just before ovulation: sustained high estrogen stimulates GnRH and LH release instead, producing the LH surge — the classic exam example of positive feedback.
Practice MCQs
Q1. Ovulation in a 28-day menstrual cycle occurs on about:
- Day 5
- Day 10
- Day 14
- Day 21
Answer: C — The LH surge triggers ovulation mid-cycle, around day 14.
Q2. The LH surge is directly responsible for:
- Follicle growth
- Ovulation
- Endometrial shedding
- Corpus albicans formation
Answer: B — The sudden LH peak ruptures the Graafian follicle and releases the secondary oocyte.
Q3. Progesterone during the luteal phase is secreted by the:
- Graafian follicle
- Corpus luteum
- Anterior pituitary
- Endometrium
Answer: B — The corpus luteum secretes progesterone during the luteal phase.
Q4. Menstruation is caused by a fall in the levels of:
- FSH and LH
- Estrogen and progesterone
- Oxytocin and prolactin
- GnRH and TSH
Answer: B — Degeneration of the corpus luteum drops estrogen/progesterone, so the endometrium breaks down.
Q5. Which is an example of positive feedback in the menstrual cycle?
- Progesterone inhibiting LH in the luteal phase
- Estrogen triggering the LH surge before ovulation
- Low estrogen stimulating FSH at cycle start
- hCG maintaining the corpus luteum
Answer: B — High estrogen stimulates (rather than inhibits) LH release, the classic positive-feedback example.
Q6. GnRH is secreted by the ____ and acts to ____.
- Anterior pituitary; stimulate the ovaries directly
- Ovaries; stimulate the anterior pituitary
- Corpus luteum; inhibit further follicle growth
- Hypothalamus; stimulate the anterior pituitary to release FSH and LH
Answer: D — GnRH from the hypothalamus drives the anterior pituitary to release FSH and LH, which drive the ovarian cycle.
Q7. Menarche and menopause:
- Mark the start and end of reproductive life, at about puberty and 45–50 years
- Both occur at puberty
- Both occur at about 45–50 years
- Mark ovulation and fertilization respectively
Answer: A — Menarche (first menstruation, around puberty) opens reproductive life; menopause (around 45–50 years) closes it.
Q8. After ovulation, the released secondary oocyte survives for about:
- One week
- 72 hours
- 24 hours
- One month
Answer: C — The released egg survives only about 24 hours, so fertilization must occur quickly after ovulation.
Q9. If fertilization occurs, the corpus luteum is kept alive by:
- Progesterone from the placenta
- Estrogen from the follicles
- The LH surge
- hCG from the implanting embryo
Answer: D — The embryo's hCG (human chorionic gonadotropin) rescues the corpus luteum, so progesterone stays high and the uterine lining is maintained.
Q10. In the follicular phase, several primary follicles begin to develop but usually only one matures into a Graafian follicle; the rest:
- Undergo atresia
- Become corpora lutea
- Release their eggs together
- Turn directly into corpus albicans
Answer: A — Under rising FSH several follicles start developing, but usually only one dominant follicle matures while the rest undergo atresia.
Q11. The luteal (secretory) phase, days 15–28, is dominated by:
- FSH
- Estrogen
- Progesterone
- GnRH
Answer: C — The corpus luteum secretes large amounts of progesterone, which thickens the endometrium and makes its glands secretory.
Q12. Which of the following is NOT true of the proliferative (follicular) phase?
- FSH stimulates the growth of ovarian follicles
- Progesterone is the dominant hormone
- Estrogen regenerates and thickens the endometrium
- Several primary follicles begin to develop
Answer: B — Progesterone dominates the luteal phase, not the follicular phase; the proliferative phase is driven by estrogen under rising FSH.
Short Questions
Q1. On which day of a 28-day cycle does ovulation occur, and what triggers it? See answer
Ovulation occurs around day 14 of a standard 28-day cycle. It is triggered by the LH surge — a sudden, massive release of luteinizing hormone from the anterior pituitary. The surge ruptures the mature Graafian follicle, releasing the secondary oocyte into the fallopian tube.
The LH surge itself is caused by positive feedback: the mature follicle secretes very high levels of estrogen, which — instead of inhibiting the pituitary as it does earlier in the cycle — stimulates the hypothalamus and pituitary to release a flood of LH. This switch from negative to positive feedback is one of the most frequently examined mechanisms in reproductive physiology.
For the exam, remember the chain in order: high estrogen → positive feedback → LH surge → follicle rupture → ovulation (day 14). A smaller FSH surge accompanies the LH surge and assists follicular rupture. The released secondary oocyte survives only about 24 hours, so fertilization must occur quickly — a detail examiners often pair with this question.
Q2. Describe the four phases of the menstrual cycle with their day ranges and dominant hormones. See answer
The cycle has four phases. The menstrual phase (days 1–5) is the shedding of the endometrium after estrogen and progesterone crash. The follicular or proliferative phase (days 1–13) follows: rising FSH grows the ovarian follicle, and the follicle's estrogen regenerates and thickens the endometrium.
The ovulatory phase (around day 14) is the brief mid-cycle event in which the LH surge ruptures the Graafian follicle and releases the secondary oocyte. Then comes the luteal or secretory phase (days 15–28): the ruptured follicle becomes the corpus luteum, which secretes progesterone to keep the endometrium thick and glandular, ready for implantation.
The memory hook is the hormone handover: estrogen runs the first half, progesterone runs the second half. Presenting the answer as a small table (phase, days, dominant hormone, ovarian and uterine events) earns full marks, because it shows the examiner both cycles — ovarian and uterine — in one view. Compare with the spermatogenesis vs oogenesis article to see how the female gamete is produced on this strict monthly timetable while sperm are made continuously.
Q3. What is the corpus luteum, and what happens to it with and without fertilization? See answer
The corpus luteum ("yellow body") is the transformed remains of the ruptured Graafian follicle after ovulation. Under the influence of LH it becomes a temporary endocrine gland that secretes large amounts of progesterone (and some estrogen). Progesterone thickens the endometrium, makes its glands secretory, and suppresses FSH and LH by negative feedback so no new follicle develops.
If fertilization occurs, the implanting embryo secretes hCG (human chorionic gonadotropin), which rescues the corpus luteum. It keeps producing progesterone for the first weeks of pregnancy, maintaining the uterine lining — and hCG is the hormone detected by pregnancy tests. The placenta later takes over progesterone production.
If fertilization does not occur, the corpus luteum has a fixed lifespan of about 14 days and then degenerates into a white fibrous scar called the corpus albicans. Estrogen and progesterone crash, the endometrium can no longer be maintained, and menstruation begins — restarting the cycle. The corpus luteum → corpus albicans sequence is an examiner favourite.
Q4. Why does menstruation occur? See answer
Menstruation occurs because the uterus prepared for a pregnancy that never arrived. Throughout the luteal phase, progesterone from the corpus luteum kept the endometrium thick, vascular and secretory, ready to receive a blastocyst. When no embryo implants, no hCG is produced to rescue the corpus luteum.
The corpus luteum therefore degenerates into the corpus albicans, and its estrogen and progesterone output collapses. The endometrium depends on these hormones for its blood supply and structural integrity: deprived of them, its spiral arteries constrict, the functional layer becomes ischemic, and it breaks down — shed along with blood through the vagina over about 4–5 days.
Only the thin basal layer of the endometrium survives; it regenerates the lining under estrogen in the next follicular phase. The hormone crash also lifts inhibition on the pituitary, so FSH rises and a new cycle begins — menstruation is simultaneously the end of one cycle and the hormonal starting gun of the next.
Q5. Differentiate between negative and positive feedback in the menstrual cycle, with examples. See answer
Negative feedback operates through most of the cycle: rising levels of estrogen and progesterone inhibit the release of GnRH from the hypothalamus and FSH/LH from the anterior pituitary. Examples: estrogen holding FSH and LH down during the follicular phase, and progesterone strongly suppressing FSH and LH during the luteal phase so that no new follicles develop while the lining is maintained.
Positive feedback occurs once per cycle, just before ovulation. When the mature Graafian follicle secretes sustained very high estrogen, the hypothalamus–pituitary response flips: instead of inhibiting LH, estrogen now stimulates GnRH and LH release. The result is the LH surge that triggers ovulation — the standard textbook example of positive feedback in human physiology.
The distinction to write in the exam: negative feedback keeps hormones balanced and prevents extra follicles; positive feedback creates the single dramatic LH surge. Examiners specifically test whether you know that the same hormone (estrogen) inhibits at moderate levels and stimulates at peak levels — that switch is the heart of hormonal regulation of the cycle.
Q6. Define menarche and menopause. How does cycle length normally vary? See answer
Menarche is the first menstrual bleeding, occurring around puberty (typically 12–13 years). It marks the beginning of reproductive life, although early cycles are often irregular and anovulatory. Menopause is the permanent cessation of menstruation, occurring around 45–50 years when the ovaries exhaust their follicles and estrogen production falls permanently.
Normal adult cycles range from 21 to 35 days, and almost all of this variation comes from the follicular phase — it lengthens or shortens depending on how long follicle maturation takes. The luteal phase is nearly constant at about 14 days in almost all women, because the corpus luteum has a fixed lifespan of roughly two weeks.
This constancy is clinically and academically useful: counting back 14 days from the next expected period estimates the ovulation date even in irregular cycles. The absence of periods outside pregnancy is called amenorrhea, and it can signal hormonal disorders, extreme weight loss or excessive exercise — a common one-line exam definition.
References
- OpenStax, Biology 2e, Chapter 43.4: Hormonal Control of Human Reproduction. https://openstax.org/books/biology-2e/pages/43-4-hormonal-control-of-human-reproduction
- NCBI Bookshelf, Endotext: The Normal Menstrual Cycle and the Control of Ovulation. https://www.ncbi.nlm.nih.gov/books/NBK279054/
- Hickman, C.P., Roberts, L.S. & Larson, A. Integrated Principles of Zoology, 18th ed. McGraw-Hill. (Reproduction; menstrual cycle.)
- Miller, S.A. & Harley, J.P. Zoology, McGraw-Hill. (Human reproduction; hormonal control.)
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