Question
Describe the phases of the menstrual cycle. Explain how the hormones FSH, LH, estrogen, and progesterone regulate each phase.
(NCERT Class 12, very high-frequency NEET question)
Solution — Step by Step
The menstrual cycle averages 28 days and involves coordinated changes in the ovary and uterus, regulated by hormones from the hypothalamus, anterior pituitary, and ovary. It has four overlapping phases.
The uterine lining (endometrium) breaks down and is shed along with blood through the vagina. This happens because progesterone and estrogen levels drop sharply (due to degeneration of the corpus luteum from the previous cycle).
FSH begins to rise toward the end of this phase, starting the next round of follicle development.
FSH (from the anterior pituitary) stimulates several ovarian follicles to grow. One dominant follicle matures into a Graafian follicle.
The growing follicle secretes increasing amounts of estrogen, which:
- Stimulates the endometrium to thicken and become richly vascularised (proliferative changes)
- Inhibits FSH secretion (negative feedback) — so only one follicle dominates
- At very high levels, triggers a surge of LH (positive feedback)
The LH surge (a sharp spike in LH levels) triggers ovulation — the mature Graafian follicle ruptures and releases the secondary oocyte (egg) into the fallopian tube.
This LH surge is triggered by the high estrogen levels from the mature follicle. Ovulation typically occurs around Day 14 of a 28-day cycle.
After ovulation, the ruptured follicle transforms into the corpus luteum under the influence of LH. The corpus luteum secretes large amounts of progesterone (and some estrogen), which:
- Maintains the thickened endometrium and makes it secretory (ready for implantation)
- Inhibits FSH and LH secretion (negative feedback) — preventing new follicle development
- Raises body temperature slightly (used in fertility tracking)
If fertilisation does NOT occur, the corpus luteum degenerates after ~10 days. Progesterone and estrogen levels plummet, the endometrium breaks down, and menstruation begins — starting the next cycle.
If fertilisation DOES occur, the embryo secretes hCG (human chorionic gonadotropin), which maintains the corpus luteum, keeping progesterone levels high and preventing menstruation.
Why This Works
The menstrual cycle is a beautiful example of hormonal feedback loops. Estrogen switches from negative to positive feedback on LH depending on its concentration — low levels inhibit LH, but sustained high levels trigger the LH surge. This switch ensures ovulation happens only when a follicle is fully mature.
Progesterone dominates the second half of the cycle, preparing the uterus for potential pregnancy and simultaneously preventing a new cycle from starting.
NEET frequently asks: “Which hormone is responsible for the LH surge?” Answer: high levels of estrogen from the mature Graafian follicle (positive feedback). Also commonly tested: hCG is the basis of pregnancy tests — it appears in urine only when an embryo implants.
Common Mistake
The biggest confusion: students mix up which hormone does what. Here is the clear division — FSH = follicle growth, LH = ovulation trigger + corpus luteum formation, Estrogen = endometrium proliferation + LH surge trigger, Progesterone = endometrium maintenance + pregnancy support.
Another trap: writing that ovulation always occurs on Day 14. This is only true for a textbook 28-day cycle. In reality, cycle length varies, and ovulation occurs ~14 days BEFORE the next period (not 14 days after the last). For a 35-day cycle, ovulation would be around Day 21.