The Menstrual Cycle

What this lesson covers in the specification

Incandio is aligned to this specification. It is not published by, endorsed by or affiliated with Pearson, and it reproduces none of Pearson's wording — the statement numbers are given so you can check every lesson against your own copy.

• 3.9 — The roles of oestrogen and progesterone in the menstrual cycle
• 3.10 — The roles of FSH and LH in the menstrual cycle (bold B statement — Paper 2 only)

Specification section 3(a) Reproduction — also taught in Sexual and Asexual Reproduction, Flowers, Pollination and Fertilisation, Seeds and Germination, The Human Reproductive Systems.

1 · Understand it

No exam language yet. The only question this section answers is: do I actually understand what is happening?

The menstrual cycle is doing two things at once in two different places, and almost all the difficulty comes from trying to follow them as one story. In the OVARY, an egg is being matured and then released. In the UTERUS, a lining is being built up, held ready, and then either kept or shed. The hormones are what keep the two in step, so that a lining is ready at the moment an egg might arrive.

Think of it like preparing a room for a guest who may not come

Every month the room is made ready — cleaned, aired, the bed made — because a guest may arrive on a particular day. Preparing it takes about two weeks, and it is then kept ready for about two more. If nobody arrives, everything is stripped out and thrown away, and the whole preparation begins again from nothing. It looks wasteful, and it is: the alternative would be to keep a room permanently ready at permanent cost, or to have nothing ready when the guest arrived. The cycle exists because the arrival cannot be predicted and the preparation cannot be rushed.

The cycle in order, taking day 1 as the first day of bleeding

1. DAYS 1–5, MENSTRUATION. The lining of the uterus, built during the previous cycle, breaks down and is lost. FSH from the pituitary gland begins to rise.
2. FSH acts on the ovary. It causes an egg to mature inside a follicle, and it stimulates the ovary to produce OESTROGEN.
3. DAYS 5–14. Oestrogen rises, and it does two things: it causes the lining of the uterus to repair and thicken, and — as it reaches a high level — it triggers a surge of LH from the pituitary.
4. DAY 14, OVULATION. The LH surge causes the mature egg to be released from the ovary into the oviduct. This is the only day of the cycle on which an egg is available.
5. DAYS 14–28. The remains of the follicle in the ovary now produce PROGESTERONE, which MAINTAINS the thickened lining, keeping it ready to receive an embryo. Progesterone also inhibits FSH and LH, so no second egg matures.
6. IF NO EMBRYO IMPLANTS: the structure producing progesterone breaks down, so the progesterone level falls. Without progesterone to maintain it, the lining breaks down — and day 1 of the next cycle begins.
7. IF AN EMBRYO IMPLANTS: progesterone continues to be produced, so the lining is maintained, menstruation does not occur, and the pregnancy continues.

Two of those steps are worth pulling out, because they are where the marks and the understanding both sit. The first is what triggers ovulation. It is not a timer — it is the LH surge, and the LH surge is itself triggered by oestrogen reaching a high level. So the egg is released because the lining is ready, not on a fixed date; the ovary is being told that the uterus has finished preparing.

The second is what causes menstruation, and it is a fall rather than a rise. The lining does not have a lifespan and does not wear out. It is being actively held in place by progesterone, and it breaks down when that support is withdrawn. Everything about the cycle's timing follows from that: as long as progesterone is maintained, the lining stays, which is exactly what happens in pregnancy and exactly what the contraceptive pill imitates.

Who makes what, and where it acts — the part most answers get muddled

1. FSH — made by the PITUITARY GLAND, acts on the OVARY. Matures an egg, and stimulates oestrogen production.
2. LH — made by the PITUITARY GLAND, acts on the OVARY. Causes ovulation.
3. OESTROGEN — made by the OVARY, acts on the UTERUS (thickens the lining) and on the PITUITARY (triggers the LH surge).
4. PROGESTERONE — made by the OVARY, acts on the UTERUS (maintains the lining) and on the PITUITARY (inhibits FSH and LH).
5. The pattern is worth noticing: the pituitary hormones travel DOWN to the ovary, and the ovarian hormones travel BACK UP to the pituitary as well as out to the uterus. That return path is the feedback.

That feedback is the same shape as the temperature and water-balance loops from Topic 2, with one interesting exception. Progesterone inhibiting FSH is ordinary negative feedback — a response that shuts off its own cause, which here prevents a second egg from maturing while one is already on its way. But the oestrogen surge is the opposite: a rising oestrogen level causes a burst of LH, which causes ovulation. That is a rising signal producing an even bigger response, and it exists because ovulation needs to be a sharp event on one day rather than a gradual drift across a week.

2 · Grade 9 Notes

A different job from the section above. You have already understood it; this is the precise set of things to LEARN — definitions to reproduce word for word, processes in order, equations with units, and the answers that score full marks.

Learn this definition · Menstrual cycle

The approximately monthly cycle in which an egg matures and is released from an ovary, and the lining of the uterus is prepared to receive an embryo and then either maintained or shed.

Learn this definition · Ovulation

The release of a mature egg from the ovary into the oviduct, caused by a surge of LH at about day 14 of the cycle.

The four hormones — source, target and effect, in one table

The 28-day cycle in order — the sequence to reproduce

1. Days 1–5: the uterus lining breaks down and is lost (menstruation); FSH begins to rise.
2. FSH causes an egg to mature in the ovary and stimulates the ovary to produce oestrogen.
3. Days 5–14: oestrogen causes the uterus lining to repair and thicken, and its rising level triggers a surge of LH.
4. Day 14: the LH surge causes ovulation — the mature egg is released into the oviduct.
5. Days 14–28: progesterone from the remains of the follicle maintains the thickened lining and inhibits FSH and LH.
6. If no embryo implants, progesterone falls, the lining is no longer maintained and breaks down, and day 1 begins again.

Reading the four-hormone graph — what to say about each curve

• FSH: a rise early in the cycle, before ovulation — it is maturing the egg
• OESTROGEN: rises through the first half and PEAKS just before ovulation — that peak is what triggers the LH surge
• LH: flat, then a sharp SPIKE at about day 14 — the spike IS the cause of ovulation
• PROGESTERONE: low until ovulation, then high through the second half, falling sharply at the end if there is no pregnancy
• The lining of the uterus: thin at day 1, thickening to day 14, maintained to day 28, then shed — always describe it against the progesterone curve

Oestrogen against progesterone — the pair examiners rely on you confusing

A. OESTROGEN BUILDS the lining: it causes the uterus lining to repair and thicken after menstruation, in the first half of the cycle. It also triggers the LH surge.

B. PROGESTERONE MAINTAINS the lining: it holds the thickened lining in place after ovulation. When progesterone FALLS, the lining breaks down and menstruation begins.

Not this: Menstruation happens because the uterus lining has worn out or reached the end of its life.

This: The lining is actively maintained by progesterone. It breaks down when the progesterone level FALLS — a withdrawal of support, not an expiry. That is why it does not break down during pregnancy.

Model answer [4 marks]

Describe the roles of oestrogen and progesterone in the menstrual cycle. [4]

Oestrogen is produced by the ovary and causes the lining of the uterus to repair and thicken after menstruation. As its level rises it also triggers a surge of LH, which causes ovulation. Progesterone is produced by the ovary after ovulation and maintains the thickened lining of the uterus, so that it is ready to receive an embryo. When the progesterone level falls, the lining is no longer maintained and it breaks down, causing menstruation.

Model answer [3 marks]

Explain what causes ovulation, and on approximately which day of the cycle it occurs. [3]

During the first half of the cycle the level of oestrogen rises, and when it reaches a high level it triggers a surge of LH from the pituitary gland. This surge of LH causes the mature egg to be released from the ovary into the oviduct. This occurs at about day 14 of a 28-day cycle.

Model answer [3 marks]

Explain why menstruation does not occur if an embryo implants in the uterus lining. [3]

Menstruation happens when the level of progesterone falls, so that the lining of the uterus is no longer maintained and breaks down. If an embryo implants, progesterone continues to be produced, so the lining continues to be maintained. It therefore does not break down, and menstruation does not occur.

Mark-losing trap. FSH and LH come from the PITUITARY and act on the OVARY. Oestrogen and progesterone come from the OVARY.

Mark-losing trap. Ovulation is caused by the LH SURGE, not by a date. The surge is itself triggered by high oestrogen.

Mark-losing trap. Menstruation is caused by progesterone FALLING. Never write that the lining 'runs out' or 'expires'.

Mark-losing trap. Oestrogen builds the lining; progesterone maintains it. Swapping them costs marks in both halves of the answer.

Mark-losing trap. Progesterone inhibits FSH — that is what stops a second egg maturing, and it is how the contraceptive pill works.

3 · Prove it — the five questions

The five questions climb Grade 6 → Grade 7 → Grade 8 → Grade 9 → Grade 9 challenge, and are marked inside Incandio on your own device, by rule, with an authored diagnosis of the mistake you actually made. The mark schemes stay in the app so that the practice is worth doing; the questions themselves are here.

1. Grade 6 · State [1 mark] — Which hormone causes the lining of the uterus to thicken?
2. Grade 7 · Explain [2 marks] — In a 28-day cycle, why does menstruation begin at about day 28 if no embryo has implanted?
3. Grade 8 · Describe [5 marks] — Describe the roles of the four hormones of the menstrual cycle, from the start of menstruation to ovulation. Select every statement that belongs in a full-mark answer.
4. Grade 9 · Calculate [3 marks] — A woman has a menstrual cycle of 32 days. Ovulation occurs 14 days before the first day of her next period. Her last period began on day 1. Calculate the day of her cycle on which ovulation occurs.
5. 9+ · Analyse [5 marks] — A contraceptive pill contains progesterone. A woman taking it every day has a progesterone level that stays high and does not fall in the way it normally would. She does not become pregnant. Select every statement that belongs in a full-mark explanation of how the pill prevents pregnancy.

The people behind this science

Two ways into the same idea — the one who showed a chemical can command an organ, and the one who explained why the body corrects itself. Inside Incandio each of them answers knowing exactly which lesson you have just finished.

Ernest Starling — the one who showed a chemical can command an organ

This cycle is four organs holding a conversation — pituitary to ovary, ovary to uterus, ovary back to pituitary — with no nerve involved anywhere in it. Starling is the reason that is a description rather than a metaphor. He proved in 1902 that an organ can command another with every nerve between them severed, because the instruction travels in the blood, and in 1905 he gave that class of messenger its name. The return path in this cycle, where the ovary's hormones act back on the gland that stimulated it, is exactly the kind of loop he opened up.

• “How can one organ give another an instruction with no nerve between them?”
• “Why does a hormonal signal take so much longer to act than a nervous one?”
• “What happens when the organ receiving a signal sends one back?”
• “Why can a chemical released into the whole body change only one organ?”

Claude Bernard — the one who explained why the body corrects itself

Progesterone inhibiting FSH so that no second egg matures is negative feedback, and Bernard is the origin of the idea that the body is a system holding itself in balance by exactly this kind of self-correction. He argued that an animal is free of its surroundings only because it maintains a constant internal environment, and he spent his career showing that a function can be identified by removing it and watching what fails — which is precisely how each hormone in this cycle was worked out.

• “Why is it useful for a response to switch off the thing that caused it?”
• “How would you find out what a hormone does, if you could not measure it?”
• “What does it mean to say the body keeps its own conditions steady?”
• “Is a cycle that repeats every month a kind of balance, or its opposite?”

Carry on through the course

• Previous lesson: The Human Reproductive Systems
• Next lesson: The Placenta and the Amniotic Fluid
• Sexual and Asexual Reproduction — Two ways to make a new organism — one produces copies, the other produces something that has never existed before. Why any species would want both.
• Flowers, Pollination and Fertilisation — Two completely different solutions to one problem — a plant that cannot move has to get its male gamete to another plant's female gamete.
• Seeds and Germination — What a seed needs to start growing, what it lives on before it has a single leaf — and why light is not on the list.
• The Human Reproductive Systems — Two systems built around the same problem — making gametes and bringing them together — and the hormones that switch them on.
• The Placenta and the Amniotic Fluid — An exchange surface with a constraint no other one has: two bloodstreams that must trade materials without ever mixing.
• All of Biology · Biology key terms · required practicals · Incandio Science · The 4BI1 specification