Four hormones, two glands and one 28-day loop. Most students can name the hormones but lose marks on which gland makes which, and on whether a given step is positive or negative feedback. Get the graph in your head and the rest follows.
📚 What you need to know
The menstrual cycle is the series of changes leading up to and following the release of an egg. It averages 28 days and is controlled by hormones.
The uterus lining, or endometrium, thickens from about day 7 to day 28 ready to receive a fertilised egg.
Ovulation happens around day 14; the egg then travels down the oviduct.
Eggs develop inside fluid-filled follicles. After releasing the egg, the empty follicle becomes the corpus luteum.
If the egg is not fertilised, the endometrium breaks down — menstruation, roughly days 1 to 7.
The pituitary gland produces FSH and LH; the ovaries produce oestrogen (from the follicle and corpus luteum) and progesterone (from the corpus luteum).
The four hormones interact through both positive and negative feedback.
The cycle in outline
Start on day 1, the first day of bleeding. Menstruation runs to about day 7, though the number of days varies. Once it finishes, the endometrium begins to thicken again, ready for a possible implantation.
Around day 14, halfway through, one mature egg is released from its follicle in the ovary. The remains of that follicle become the corpus luteum, which is not a leftover but an active hormone-producing structure. If the egg is not fertilised, the corpus luteum breaks down, the endometrium is shed, and day 1 comes round again.
🧩 The 28 days, in order
Days 1–7 — menstruation: the endometrium breaks down and is lost through the vagina.
Days 7–14 — FSH stimulates follicles to develop; the follicle wall secretes oestrogen and the endometrium thickens.
Day 14 — oestrogen peaks, triggering a surge of LH, which causes ovulation.
Days 14–28 — the corpus luteum secretes progesterone, which maintains the thickened endometrium.
Day 28 — with no fertilisation, the corpus luteum deteriorates, progesterone falls, and the lining breaks down.
The two pituitary hormones
FSH (follicle-stimulating hormone) is secreted by the pituitary gland and stimulates several immature egg cells in follicles in the ovary to develop. It also stimulates the follicle wall to secrete oestrogen.
LH (luteinising hormone) is released by the pituitary when oestrogen levels have reached their peak. LH causes ovulation — the mature egg is shed from the follicle and released from the ovary. LH also stimulates the empty follicle to become the corpus luteum and to produce progesterone.
Notice the order: FSH acts first, oestrogen builds, then LH spikes. Ovulation is the response to that spike, not the cause of it.
The two ovarian hormones
Oestrogen rises from day 1 to a peak just before day 14. It causes the endometrium to start thickening and the egg cell to mature. The peak comes just before the egg is released, and that is not a coincidence — the peak is what triggers the LH surge.
Progesterone stays low from day 1 to 14 and starts to rise once ovulation has occurred, because it comes from the corpus luteum, which does not exist before then. Rising progesterone causes the endometrium to continue thickening. When the corpus luteum deteriorates, progesterone falls, the endometrium breaks down, and menstruation follows.
Line the two graphs up mentally: the oestrogen peak sits directly under the LH spike, because one causes the other.
Hormone
Made by
Main effects
FSH
Pituitary gland
Stimulates follicles to develop in the ovary; stimulates the follicle wall to secrete oestrogen
LH
Pituitary gland
At its peak causes ovulation; causes the follicle wall to develop into the corpus luteum, which secretes progesterone
Oestrogen
Follicle, then the corpus luteum
Thickens the endometrium and matures the egg; at high levels inhibits FSH and stimulates the release of LH
Progesterone
Corpus luteum
Maintains and thickens the endometrium; inhibits FSH and LH secretion
Positive and negative feedback
This is where most marks are won and lost, so take it slowly.
FSH and oestrogen
FSH stimulates a follicle to develop, and the follicle wall produces oestrogen — so FSH stimulates oestrogen production. Oestrogen then causes an increase in FSH receptors, making the follicles more receptive to FSH, which stimulates still more oestrogen production. That loop is positive feedback.
But once oestrogen levels are high enough, oestrogen inhibits the secretion of FSH. That is negative feedback, and it stops more follicles being recruited.
LH and oestrogen
When oestrogen rises high enough it stimulates the release of LH from the pituitary, causing ovulation around day 14. After ovulation, LH causes the follicle wall to become the corpus luteum, which secretes more oestrogen — again positive feedback.
LH and progesterone
LH also drives the corpus luteum to secrete progesterone. Progesterone thickens and maintains the endometrium, but it also inhibits the secretion of FSH and LH from the pituitary gland. That is negative feedback, and it prevents another egg maturing while the lining is being prepared.
The quick test. If a hormone makes more of itself or more of its own trigger, that is positive feedback. If it shuts down what produced it, that is negative feedback. Apply that test rather than trying to memorise the four cases separately.
Remember that hormones travel everywhere in the bloodstream but only act on their target organ. FSH reaches your toes; it simply does nothing there.
Hormones in IVF treatment
Some couples find it difficult to conceive naturally, sometimes because of insufficient levels of reproductive hormones affecting the development of egg or sperm cells, and sometimes because of a problem with the reproductive system itself. One treatment is in vitro fertilisation (IVF): eggs are fertilised by sperm outside the body under controlled laboratory conditions.
🧩 The main steps of IVF
A drug is given to inhibit FSH and LH secretion from the pituitary. Oestrogen and progesterone secretion stop too, which temporarily halts the cycle and lets doctors control the timing and quantity of egg production.
Injections of FSH and LH stimulate follicle development. Because the FSH concentration is far higher than in a normal cycle, superovulation occurs and many more follicles than usual begin to mature.
The eggs are collected and fertilised by sperm in sterile laboratory conditions.
The fertilised eggs develop into embryos. About 48 hours after fertilisation, when they are tiny balls of cells, one or more are inserted into the uterus.
Extra progesterone is normally given to make sure the endometrium is maintained.
The success rate of IVF is low, at roughly 30%, although improvements in medical technology continue to raise it.
Worked examples
WORKED EXAMPLE
Explain how the rise in oestrogen before day 14 leads to ovulation. [3]
Step 1 — where the oestrogen comes from
FSH stimulates a follicle to develop, and the follicle wall secretes oestrogen, so oestrogen levels rise through the first half of the cycle.
Step 2 — the trigger
When oestrogen reaches its peak it stimulates the pituitary gland to release a surge of LH. This is positive feedback.
Step 3 — the result
The LH surge causes ovulation: the mature egg is shed from the follicle and released from the ovary, at around day 14.
3 marksNaming it as positive feedback often picks up an extra mark on longer questions.
WORKED EXAMPLE
Explain why menstruation occurs at the end of a cycle in which the egg was not fertilised. [3]
Step 1 — what was maintaining the lining
After ovulation the corpus luteum secretes progesterone, which maintains and thickens the endometrium.
Step 2 — what changes
Without fertilisation the corpus luteum deteriorates, so progesterone levels fall.
Step 3 — the consequence
With progesterone no longer maintaining it, the endometrium breaks down and is lost through the vagina as menstrual blood.
3 marksThe falling progesterone is the cause. “The egg was not fertilised” alone is not an explanation.
💡 Exam tip
Always say which gland makes the hormone. FSH and LH from the pituitary, oestrogen and progesterone from the ovary.
Read hormone graphs by their peaks: oestrogen just before day 14, LH at day 14, progesterone around day 21.
State explicitly whether a step is positive or negative feedback — it is usually a separate mark.
The corpus luteum is the source of progesterone, so nothing about progesterone can happen before ovulation.
Use endometrium rather than “the lining” once you know the word.
For IVF, the logic is always: shut the cycle down, then control it artificially with injected hormones.
⚠ Common mix-up
LH does not cause the follicle to grow. FSH does that; LH releases the egg.
Ovulation does not cause the LH surge. The surge causes ovulation.
The corpus luteum is not the egg. It is what the follicle becomes after the egg has gone.
Day 14 is an average, not a rule. Cycle length and the day of ovulation both vary.
Oestrogen does not only build the lining. At its peak it also inhibits FSH and triggers LH.
IVF does not use progesterone to fertilise anything. It is given at the end to maintain the endometrium.
Up next: Fertilisation — what actually happens in the moments when a sperm reaches an egg, and why only the nucleus gets in.
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