Question
Explain the menstrual cycle in detail, covering the four phases and the hormonal changes (FSH, LH, oestrogen, progesterone) occurring throughout the approximately 28-day cycle.
Solution — Step by Step
The menstrual cycle is the monthly preparation of the female reproductive system for potential pregnancy. It involves coordinated changes in the ovary (ovarian cycle) and the uterus (uterine cycle), both driven by hormones from the hypothalamus (GnRH), anterior pituitary (FSH and LH), and ovary (oestrogen and progesterone).
The average cycle is 28 days, though 21–35 days is normal. Day 1 is conventionally the first day of menstrual flow.
What happens: The uterine lining (endometrium) shed. Blood and tissue flow out — this is menstruation.
Why it happens: Progesterone and oestrogen levels dropped sharply at the end of the previous cycle (because no fertilisation occurred). Without hormonal support, the thickened endometrium breaks down.
Hormones: FSH begins rising (anterior pituitary is stimulated by GnRH to secrete FSH). LH is low. Oestrogen and progesterone are at their lowest.
What happens: Several ovarian follicles start developing under FSH stimulation. One becomes dominant — the Graafian follicle. The follicle produces increasing amounts of oestrogen.
Oestrogen’s role: Rebuilds the endometrium (proliferative phase of the uterine cycle). Also feeds back to the pituitary — initially negative feedback, suppressing FSH rise. But as oestrogen levels peak (around Day 12–13), it switches to positive feedback, triggering a sudden surge in LH.
Hormones: FSH rises then levels off. Oestrogen rises steadily, peaks near Day 13. LH remains low until the surge.
What happens: The LH surge (Day 13–14) triggers ovulation — the Graafian follicle ruptures and releases the secondary oocyte. This oocyte is swept into the fallopian tube by fimbria.
This is the fertile window. Fertilisation must occur within 12–24 hours of ovulation (lifespan of the egg), though sperm can survive 3–5 days in the female reproductive tract.
Hormones: LH peaks sharply. FSH also shows a smaller peak. Oestrogen dips briefly just after ovulation.
What happens: The ruptured follicle transforms into the corpus luteum (under LH influence). The corpus luteum secretes large amounts of progesterone and some oestrogen. Progesterone maintains the thickened, glandular endometrium — ready for implantation if fertilisation occurs.
If fertilisation does not occur: the corpus luteum degenerates (by Day 26–28) → progesterone and oestrogen levels fall → endometrium cannot be maintained → menstruation begins again (Day 1 of the next cycle).
If fertilisation does occur: the embryo secretes hCG (human chorionic gonadotropin), which rescues the corpus luteum and maintains progesterone levels throughout early pregnancy.
Hormones: Progesterone high (dominant hormone). Oestrogen moderate. FSH and LH suppressed by negative feedback from progesterone.
Why This Works
The menstrual cycle is a negative and positive feedback system. Most of the time, rising oestrogen and progesterone suppress FSH and LH (negative feedback) — preventing multiple ovulations. The one exception is the brief oestrogen surge near Day 13, which temporarily switches to positive feedback and causes the LH surge that triggers ovulation. This positive feedback mechanism is a classic example in biology — one of very few in the body (milk let-down is another).
The corpus luteum lifespan (~14 days) is what makes the luteal phase relatively constant at about 14 days across women, even when the overall cycle length varies. Variation in cycle length almost always comes from variation in the follicular phase, not the luteal phase.
Alternative Method — Summary Table
| Day | Phase | Key Events | Dominant Hormone |
|---|---|---|---|
| 1–5 | Menstrual | Endometrium shed | Low E, Low P |
| 1–13 | Follicular | Follicle grows, endometrium rebuilds | FSH → Oestrogen ↑ |
| 14 | Ovulatory | LH surge → Ovulation | LH peak |
| 15–28 | Luteal | Corpus luteum forms, endometrium maintained | Progesterone |
Common Mistake
Students often say “oestrogen causes negative feedback throughout the cycle.” This is mostly true, but around Day 12–13, high oestrogen causes positive feedback, triggering the LH surge. Getting this distinction right is essential for NEET questions.
Another common error: mixing up FSH and LH roles. FSH stimulates follicle growth; LH triggers ovulation and corpus luteum formation. They are separate hormones with distinct roles, though both come from the anterior pituitary.
NEET frequently asks: (1) which hormone triggers ovulation — LH surge; (2) what maintains the corpus luteum in early pregnancy — hCG; (3) why progesterone is called the “hormone of pregnancy” — it maintains the endometrium; (4) what happens if progesterone falls at Day 26 — menstruation. These four questions together account for most of the marks on this topic.