Edexcel IGCSE Biology · Spec 3.9-3.10B
The Menstrual Cycle (Triple)
The hormones that control the menstrual cycle — FSH, LH, oestrogen and progesterone.
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The Menstrual Cycle (Triple)
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Explained
Four hormones, two glands, and the feedback that connects them
The cycle is run by four hormones from two places. FSH and LH come from the pituitary gland in the brain. Oestrogen and progesterone come from the ovary.
That split is the structure of the whole topic. The brain sends instructions down to the ovary, and the ovary reports back up to the brain, and the timing of the cycle comes out of that conversation rather than from any single hormone.
What each one does
FSH, follicle stimulating hormone, is released by the pituitary and travels to the ovary. It causes an egg to mature inside a follicle, and it stimulates the ovary to produce oestrogen.
Oestrogen, from the ovary, repairs and thickens the lining of the uterus. It also inhibits FSH, so no further eggs begin maturing, and it stimulates the release of LH.
LH, luteinising hormone, is released by the pituitary in a sharp surge at around day 14, and that surge triggers ovulation. LH releases the egg; it does not mature it.
Progesterone, from the structure left behind in the ovary after ovulation, maintains the thickened lining and inhibits both FSH and LH, so no new cycle begins while it is high.
The sequence, told as one story
FSH rises first and matures an egg. The maturing follicle produces oestrogen, so oestrogen rises through the first half. Rising oestrogen shuts FSH down and triggers a surge of LH. The LH surge causes ovulation at about day 14. The emptied follicle then produces progesterone, which maintains the lining and holds FSH and LH down. If no embryo implants, progesterone falls, the lining breaks down, and with progesterone gone FSH is free to rise again and start the next cycle.
Read that once more and notice that nothing decides when the cycle restarts except the falling of progesterone. The cycle is a loop, and the loop closes itself.
Negative feedback
Oestrogen inhibiting FSH, and progesterone inhibiting both FSH and LH, are negative feedback. A hormone produced downstream suppresses the hormone that caused it.
This is what stops several eggs maturing at once, and it is how the contraceptive pill works. Keeping oestrogen and progesterone artificially high holds FSH and LH permanently suppressed, so no egg matures and no ovulation occurs.
The exception is worth knowing, because it is unusual in this course. Oestrogen stimulating LH is positive feedback: a rising level causing a further rise. It is what produces the sharp LH spike rather than a gentle increase, and a spike is what ovulation needs.
What examiners say about this topic
A principal examiner report for Edexcel International GCSE Biology describes a question testing FSH in an unfamiliar context, where candidates had to say what a reduction in FSH would mean. Most correctly said that fewer eggs or follicles would mature.
The report then works through the answers that failed, and one is worth reading twice. It records a response that gained no credit despite the candidate having appreciated that FSH targets the ovaries, because it did not state that FSH stimulates egg development or maturation.
Knowing where a hormone goes is not knowing what it does. FSH, LH, oestrogen and progesterone all act on the reproductive system, so naming the target organ distinguishes none of them. The effect is the answer.
The report also notes another response gaining nothing from a candidate who understood the roles of the different hormones but had confused which role belonged to which. That is the characteristic failure in this topic: the content is known, and it is attached to the wrong name.
Two habits protect against it. Learn each hormone as a sentence with a verb in it, so FSH matures the egg, LH releases it, oestrogen thickens the lining, progesterone maintains it. And learn the two pituitary hormones as a pair, since FSH and LH are the ones most often swapped.
Reading the four graphs
Questions here usually give all four hormone levels plotted against the day of the cycle, and each curve has a shape you can recognise.
FSH is highest early and low afterwards. Oestrogen climbs steadily to a peak just before day 14. LH is flat apart from one sharp spike at day 14. Progesterone is low until day 14, then rises to a broad peak and falls away at the end.
The spike is the easiest landmark. Find it, call it LH, call that point ovulation, and the rest of the graph is oriented around it.
Spec 3.9-3.10B
What you need to know
- Describe the events of the menstrual cycle
- Explain the roles of the hormones FSH and LH
- Explain the roles of oestrogen and progesterone
Active recall
Quick check
Answer each question before opening the answer.
What are the roles of oestrogen and progesterone in the cycle?
Oestrogen repairs and thickens the uterus lining and triggers LH; progesterone maintains the lining.
What do FSH and LH do?
FSH causes an egg to mature and stimulates oestrogen; LH triggers ovulation.
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