Four hormones, two organs and one graph that turns up in Paper 2 again and again. The mistake almost everyone makes is learning the hormone levels as a shape to memorise. Learn them as a chain of causes instead and you can rebuild the whole graph from the first hormone.
📘 What you need to know
The cycle averages 28 days. Day 1 is the first day of menstruation, and ovulation happens around day 14.
Two hormones come from the pituitary gland: FSH and LH. Two come from the ovary: oestrogen and progesterone.
FSH stimulates a follicle to develop and to secrete oestrogen. Oestrogen repairs and thickens the endometrium.
When oestrogen peaks it triggers a surge of LH, and the LH surge causes ovulation. The empty follicle becomes the corpus luteum.
The corpus luteum secretes progesterone, which maintains the endometrium and inhibits FSH and LH.
If there is no implantation the corpus luteum breaks down, progesterone falls, and the endometrium is shed — menstruation, and the start of the next cycle.
The cycle uses both positive feedback (oestrogen driving the LH surge) and negative feedback (progesterone inhibiting FSH and LH).
Two jobs running at the same time
The cycle is easier to hold in your head once you notice it is doing two separate things in parallel. In the ovary, an egg is being matured and then released. In the uterus, a lining is being built in case that egg is fertilised. The hormones are simply the messages that keep the two in step, so that the lining is at its thickest at exactly the moment an embryo might arrive.
Why the timing matters. A fertilised egg takes about a week to reach the uterus and implant. So the lining has to peak after ovulation, not before. Every feature of the second half of the cycle exists to hold the lining ready for that window.
The four hormones
Hormone
Made by
What it does
FSH (follicle stimulating hormone)
Pituitary gland
Stimulates immature egg cells in follicles to develop, and stimulates the follicle wall to secrete oestrogen
LH (luteinising hormone)
Pituitary gland
A surge of LH triggers ovulation, then causes the empty follicle to become the corpus luteum
Oestrogen
Follicle wall, then the corpus luteum
Repairs and thickens the endometrium; at high levels it triggers the LH surge
Progesterone
Corpus luteum
Maintains the thickened endometrium and inhibits FSH and LH so no new follicle develops
Notice that only the ovary makes oestrogen and progesterone. That means if the ovary is not producing a follicle or a corpus luteum, those hormones cannot appear. Several exam questions are solved just by asking “which structure is secreting this, and does it exist yet?”
The graph, and how to read it
Below is the whole cycle on one set of axes: the four hormones on top, the thickness of the uterus lining underneath. Keeping them aligned is the point — every change in the lining has a hormone change directly above it.
The LH spike is narrow on purpose: it lasts roughly a day. If you are asked to identify ovulation from an unlabelled graph, the sharp single peak is the giveaway.
🧩 Rebuilding the graph from scratch
FSH rises first. It stimulates follicles to develop. Nothing else can happen until this does.
Oestrogen climbs as the growing follicle wall secretes it. The endometrium starts to repair and thicken.
Oestrogen peaks just before day 14 and triggers a sharp LH surge.
LH causes ovulation and turns the empty follicle into the corpus luteum.
Progesterone rises from the corpus luteum, holding the lining thick and inhibiting FSH and LH.
No implantation means the corpus luteum degenerates, progesterone falls, the lining breaks down, and FSH is released from inhibition — so the next cycle begins.
The feedback, which is where the marks hide
This is the part that separates a grade 5 from a grade 7. The same hormone can do opposite things depending on its level, and examiners ask about it precisely because it is counter-intuitive.
Oestrogen appears on both sides of the story. At moderate levels it inhibits FSH; at its peak it stimulates the LH surge. That switch is the single most examined subtlety in this topic.
Positive feedback
Oestrogen also increases the number of FSH receptors on the follicles. More receptors means the follicles respond more strongly to FSH, which makes them secrete still more oestrogen. The output amplifies its own cause — that is the definition of positive feedback. It builds until oestrogen is high enough to trigger the LH surge.
Negative feedback
Once progesterone is being produced by the corpus luteum, it inhibits the secretion of FSH and LH from the pituitary. Low FSH means no new follicle develops, so a second egg is not released while the body is preparing for a possible pregnancy. Oestrogen at moderate levels inhibits FSH in the same way.
When no fertilisation happens
If the egg is not fertilised, no embryo implants, and the corpus luteum has nothing to sustain it. It degenerates, so progesterone production stops. The lining can only be maintained while progesterone is present, so it breaks down and is lost through the vagina as menstrual blood, typically over days 1 to 7.
The falling progesterone has a second consequence: with the inhibition removed, FSH can rise again. That is why menstruation and the start of a new follicle happen at the same time.
Using the same hormones: IVF
In vitro fertilisation is worth knowing because it is the cycle run deliberately rather than naturally. A drug first inhibits FSH and LH, which switches the natural cycle off and hands control of the timing to the doctors. Injections of FSH and LH then stimulate many follicles at once, so far more eggs mature than in a normal cycle. The eggs are collected and fertilised with sperm in the laboratory, the resulting embryos are grown for a couple of days, and one or more are transferred into the uterus. Extra progesterone is usually given afterwards, for the same reason it matters in a normal cycle: without it, the lining cannot be maintained.
Worked examples
WORKED EXAMPLE
Explain why the concentration of LH rises sharply around day 14 of the cycle. [3]
Work backwards from the surgewhat is high just before day 14? oestrogenBuild the chainFSH has stimulated the follicle, whose wall secretes increasing amounts of oestrogenoestrogen concentration reaches a peak just before day 14at this high level oestrogen stimulates the pituitary to release LH — positive feedback3 marks: rising oestrogen from the follicle, peak level reached, positive feedback on the pituitarysaying “LH causes ovulation” answers a different question — here they asked what causes the LH
WORKED EXAMPLE
A contraceptive pill contains progesterone. Suggest how this prevents pregnancy. [2]
Recall what progesterone normally does to the pituitaryit inhibits FSH and LH — that is negative feedbackFollow the consequencekeeping progesterone high means FSH stays low, so no follicle develops and no egg maturesLH also stays low, so there is no surge and therefore no ovulation1 mark for inhibition of FSH and LH, 1 mark for no follicle development or no ovulation“suggest” again — you are applying the feedback loop, not recalling a fact about the pill
💡 Exam tips
Learn the order of causes, not the shapes: FSH → oestrogen → LH surge → ovulation → progesterone. Everything else can be reconstructed from that.
When a graph question gives you an axis with no labels, find the narrow spike — that is LH, and it puts you at day 14 immediately.
Always name the source organ as well as the hormone. “Progesterone from the corpus luteum” is a stronger answer than “progesterone”.
For any “what happens if X is blocked” question, trace forwards along the chain and state each broken step in order.
Menstruation is caused by progesterone falling, not by oestrogen falling and not by LH. This is worth checking every time.
⚠️ Common mix-ups
Swapping FSH and LH. FSH develops the Follicle; LH is the one that Launches the egg. Use the initial letters.
Saying oestrogen causes ovulation. Oestrogen triggers the LH surge; LH causes ovulation. That is two steps, and both are worth marks.
Thinking oestrogen only ever does negative feedback. At its peak it switches to positive feedback. Say which level you mean.
Placing the thickest lining at ovulation. The lining peaks after ovulation, in the second half of the cycle, ready for implantation.
Calling the corpus luteum an egg. It is what is left of the follicle after the egg has gone, and its whole job is secreting progesterone.
Writing that menstruation happens because the egg dies. It happens because the corpus luteum degenerates and progesterone falls.
Up next: Fertilisation — what actually happens in the few minutes when a sperm reaches the egg, and why only one of the millions gets in.
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