Menstrual Cycle and Hormones Explained: FSH, LH, Estrogen and Progesterone for NEET
Every year NEET asks some version of one question: which hormone peaks when, and what does it trigger? Students who memorise a 28-day chart blank out the moment the question is reworded, while students who understand the feedback loop answer it in ten seconds.
Quick Summary
The menstrual cycle has four phases: menstrual, follicular (proliferative), ovulatory and luteal (secretory).
FSH grows the follicle, a mid-cycle LH surge triggers ovulation, and progesterone from the corpus luteum maintains the endometrium.
Menstruation happens because progesterone and estrogen fall when the corpus luteum degenerates.
If fertilisation occurs, hCG keeps the corpus luteum alive, so progesterone stays high and the cycle pauses.
What Is the Menstrual Cycle?
The menstrual cycle is the repeating series of changes in the ovary and uterus that prepares the body for a possible pregnancy. In a typical human female it lasts about 28 days, with ovulation around day 14. Cycle length varies between individuals, but NEET questions almost always use the 28-day model.
Two organs run in parallel: the ovary (follicle growth, ovulation, corpus luteum) and the uterus (endometrium building up and breaking down). Hormones from the pituitary and ovary keep the two in step.
The Four Phases at a Glance
Think of the cycle as one story in four chapters, each with a single dominant hormone.
Menstrual phase (days 1-5): the endometrium breaks down and is shed with blood. Estrogen and progesterone are both low.
Follicular phase (days 6-13): the primary follicle matures into a Graafian follicle under FSH. Rising estrogen rebuilds the endometrium, which is why this is also called the proliferative phase.
Ovulatory phase (around day 14): the Graafian follicle ruptures and releases the secondary oocyte after the LH surge.
Luteal phase (days 15-28): the ruptured follicle becomes the corpus luteum, which secretes progesterone. The endometrium becomes thick, vascular and glandular, hence the name secretory phase.
The Hormone Logic: Who Triggers Whom
The hypothalamus releases GnRH, which makes the anterior pituitary release FSH and LH. FSH stimulates follicle growth, and the growing follicle's granulosa cells secrete estrogen.
Estrogen first suppresses FSH slightly, but once it crosses a high threshold it switches to positive feedback on the pituitary. That produces the mid-cycle LH surge, with a smaller FSH surge alongside it, and ovulation follows roughly a day later.
Key Insight: The only positive feedback loop in the normal cycle is estrogen triggering the LH surge. Everything else is negative feedback. Students who draw this one loop on the first line of the answer sheet tend to get the whole hormone graph right without memorising it, which is how it is usually taught at Sukriti NeetJee.
What Happens After Ovulation
After ovulation, LH converts the empty follicle into the corpus luteum. It secretes large amounts of progesterone and some estrogen, which maintain the thickened endometrium and suppress new FSH and LH.
If the egg is not fertilised, the corpus luteum degenerates after about 10-12 days. Progesterone and estrogen then collapse, the endometrium loses support, and menstruation begins. The cycle restarts.
If fertilisation and implantation occur, the placenta secretes hCG, which rescues the corpus luteum. Progesterone stays high, the endometrium is retained and menstruation does not occur.
Oogenesis Link: Where the Arrests Happen
NEET loves to join the cycle to oogenesis. A primary oocyte is formed before birth and stays arrested in prophase I until puberty. Just before ovulation, meiosis I completes and a secondary oocyte arrested at metaphase II is released.
Meiosis II completes only if a sperm penetrates the oocyte. So the egg that is ovulated is technically a secondary oocyte, not a mature ovum.
Common NEET Traps
Confusing the follicular phase (ovary label) with the proliferative phase (uterus label). They are the same days seen from two organs.
Saying progesterone peaks at ovulation. It peaks in the mid-luteal phase.
Assuming menstruation is caused by high hormone levels. It is caused by a sudden fall.
Forgetting that LH, not FSH, is the main trigger of ovulation.
Frequently Asked Questions
Which hormone causes ovulation?
The surge of luteinising hormone (LH) from the anterior pituitary causes the Graafian follicle to rupture and release the secondary oocyte. The surge is triggered by high estrogen levels acting through positive feedback.
Why does menstruation occur?
If fertilisation does not happen, the corpus luteum degenerates and progesterone and estrogen levels fall sharply. The endometrium loses hormonal support, breaks down and is shed as menstrual flow.
What is the role of the corpus luteum?
The corpus luteum forms from the ruptured follicle after ovulation and secretes progesterone. This hormone maintains the thick, glandular endometrium needed for implantation.
What is the difference between follicular and proliferative phases?
They cover the same days. Follicular describes what happens in the ovary, where follicles grow, while proliferative describes what happens in the uterus, where the endometrium thickens under estrogen.
What does hCG do in early pregnancy?
Human chorionic gonadotropin, secreted by the placenta, keeps the corpus luteum functioning so that progesterone remains high. This prevents menstruation and sustains the early pregnancy.
Over to You
Which hormone graph still confuses you? Leave your NEET prep questions in the comments, and share this post with a fellow aspirant who is revising reproduction.
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