Question
Describe the phases of the menstrual cycle and explain how FSH, LH, estrogen, and progesterone regulate each phase.
Solution — Step by Step
The menstrual cycle lasts approximately 28 days and has four phases: menstrual phase (days 1–5), follicular phase (days 1–13), ovulatory phase (day 14), and luteal phase (days 15–28). Note that the menstrual and follicular phases overlap — day 1 marks both menstruation and the start of follicle development.
The hypothalamus releases GnRH, which signals the anterior pituitary to secrete FSH (Follicle Stimulating Hormone). FSH stimulates the primary follicles in the ovary to grow and mature. As the follicle grows, it secretes increasing amounts of estrogen, which causes the endometrium (uterine lining) to thicken — this is called the proliferative phase of the uterus.
Why does ovulation happen at day 14 specifically? By the end of the follicular phase, estrogen levels peak. High estrogen switches from negative feedback to positive feedback on the pituitary — this is a crucial reversal that most students miss. The LH surge (sharp spike in LH) ruptures the Graafian follicle, releasing the secondary oocyte. This is ovulation.
After ovulation, the ruptured follicle collapses and transforms into the corpus luteum (yellow body). Under the influence of LH, the corpus luteum secretes large amounts of progesterone (and some estrogen). Progesterone converts the thickened endometrium into a secretory state — rich in glycogen, blood vessels, and ready to receive an embryo.
If fertilisation does not occur, the corpus luteum degenerates by day 26, forming the corpus albicans (white body). Progesterone and estrogen levels fall sharply. Without progesterone support, the endometrium sheds — this is menstruation. The low hormone levels now remove the feedback inhibition, so FSH rises again and the next cycle begins.
Why This Works
The entire cycle is a beautifully coordinated hormonal cascade. FSH builds the follicle; the follicle makes estrogen; estrogen primes the uterus AND eventually triggers the LH surge; LH causes ovulation and then builds the corpus luteum; progesterone prepares the uterus for implantation.
The feedback switch from negative to positive is what drives the cycle forward rather than just suppressing itself. At low-to-moderate estrogen levels, the pituitary reduces FSH output (negative feedback — prevents multiple follicles from maturing simultaneously). But at the peak estrogen concentration seen just before ovulation, the same pituitary suddenly releases a flood of LH instead — positive feedback triggers the surge.
This pattern — one hormone triggering the next in a relay — is what NEET questions most frequently test. The sequence FSH → estrogen → LH surge → ovulation → corpus luteum → progesterone is the backbone answer for almost every variation of this question.
Alternative Method — Tabular Summary
For last-minute revision, memorise this table:
| Phase | Days | Key Hormone | Uterine Change |
|---|---|---|---|
| Menstrual | 1–5 | Estrogen & progesterone drop | Endometrium sheds |
| Follicular | 1–13 | FSH → Estrogen ↑ | Endometrium proliferates |
| Ovulatory | 14 | LH surge | — |
| Luteal | 15–28 | LH → Progesterone ↑ | Endometrium becomes secretory |
NEET frequently asks: “Which hormone is responsible for the LH surge?” The answer is high estrogen (positive feedback). Do not write “GnRH” — that’s technically upstream but not the direct trigger being asked about in the options.
Common Mistake
Students often write that progesterone is secreted by the follicle. Wrong. The follicle secretes estrogen. Progesterone is secreted by the corpus luteum — the structure that forms after the follicle ruptures at ovulation. Getting this mixed up costs marks in both NEET and CBSE board 3-mark questions. Remember: follicle → estrogen; corpus luteum → progesterone.
A second trap: many students think menstruation happens because progesterone rises. The opposite is true — menstruation is caused by the fall in progesterone (and estrogen) when the corpus luteum degenerates. The drop in support is what causes the endometrium to shed.