Hormones in Human Reproduction

Hormones do far more than control glucose and temperature. They also drive the great changes of puberty and run the menstrual cycle, and we can use our knowledge of them both to prevent pregnancy (contraception) and to treat infertility. This lesson covers the hormones of reproduction: testosterone and oestrogen at puberty, the four hormones of the menstrual cycle (FSH, LH, oestrogen and progesterone), how those hormones can be controlled with contraception, and how they can be used to help people who are struggling to conceive. This is part of Topic B3 of OCR Gateway Combined Science A.

By the end of this lesson you should be able to describe the role of hormones at puberty, name the four menstrual-cycle hormones and their roles, explain how the hormones interact to control the cycle, compare hormonal and barrier methods of contraception, and evaluate the use of hormones to treat infertility.

This lesson develops all three AOs: AO1 (recalling the four menstrual-cycle hormones and their roles), AO2 (applying that knowledge to how contraception works), and AO3 (evaluating the benefits and drawbacks of using hormones to treat infertility).

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Hormones at Puberty

During puberty, the body develops the secondary sexual characteristics — the physical changes of becoming an adult capable of reproduction. These changes are triggered by reproductive hormones.

• In males, the main reproductive hormone is testosterone, produced by the testes. It stimulates sperm production and the male secondary sexual characteristics (such as facial and body hair, a deeper voice, and muscle growth).
• In females, the main reproductive hormone is oestrogen, produced by the ovaries. It stimulates the female secondary sexual characteristics (such as breast development and the widening of the hips) and the start of the menstrual cycle.

So testosterone is the principal male hormone and oestrogen the principal female hormone, each controlling the changes of puberty in its sex. Secondary sexual characteristics are the features that develop at puberty; they are different from the primary sexual characteristics — the reproductive organs themselves, which are present from birth. These changes happen gradually over several years, a slow and long-lasting process that suits hormonal rather than nervous control.

Exam Tip: Pair the hormone with its gland and sex: testosterone – testes – male; oestrogen – ovaries – female. A common slip is to write that testosterone is made by the pituitary; it is made by the testes.

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The Menstrual Cycle

The menstrual cycle is a roughly 28-day cycle in females that prepares the body for a possible pregnancy each month. It is controlled by four hormones released by the pituitary gland and the ovaries.

The cycle has key events:

• Days 1–4: menstruation — the lining of the uterus breaks down and is lost (a "period").
• Around day 5–13: the uterus lining builds up again, becoming thick and full of blood vessels, ready to receive a fertilised egg.
• Around day 14: ovulation — an egg is released from the ovary.
• Days 15–28: the lining is maintained; if no fertilised egg implants, the lining breaks down and the cycle begins again.

The four hormones

Hormone	Released by	Main role
FSH (follicle-stimulating hormone)	Pituitary gland	Causes an egg to mature in the ovary; stimulates the ovaries to release oestrogen
Oestrogen	Ovaries	Causes the uterus lining to build up; stimulates the release of LH and inhibits FSH
LH (luteinising hormone)	Pituitary gland	Triggers ovulation (the release of the egg) at about day 14
Progesterone	Ovaries	Maintains the uterus lining in the second half of the cycle; inhibits FSH and LH

The diagram shows the cycle as a timeline: menstruation first, then the lining building up under oestrogen, then ovulation triggered by LH at about day 14, then the lining maintained by progesterone until day 28.

How the hormones interact

The four hormones control each other in a feedback loop that drives the cycle forward:

1. FSH (from the pituitary) makes an egg mature and stimulates the ovaries to produce oestrogen.
2. Rising oestrogen causes the uterus lining to thicken, and it inhibits FSH (so no more eggs mature) while stimulating the release of LH.
3. A surge of LH (from the pituitary) triggers ovulation at about day 14.
4. After ovulation, the ovary releases progesterone, which maintains the uterus lining and inhibits both FSH and LH.
5. If the egg is not fertilised, progesterone (and oestrogen) levels fall, the lining breaks down (menstruation), the inhibition of FSH is removed, and the cycle starts again.

flowchart TD
  A["FSH (pituitary)<br/>matures egg, stimulates oestrogen"] --> B["Oestrogen (ovary)<br/>builds lining, inhibits FSH, stimulates LH"]
  B --> C["LH (pituitary)<br/>triggers ovulation (~day 14)"]
  C --> D["Progesterone (ovary)<br/>maintains lining, inhibits FSH and LH"]
  D -.->|"if no fertilisation, levels fall,<br/>lining breaks down, cycle restarts"| A

Exam Tip: Keep the two pituitary hormones and the two ovary hormones straight. FSH and LH come from the pituitary; oestrogen and progesterone come from the ovaries. FSH matures the egg; LH triggers ovulation; oestrogen builds the lining; progesterone maintains it.

What happens if the egg is fertilised

The whole point of the menstrual cycle is to prepare the uterus for a possible pregnancy. If the released egg is fertilised by a sperm and the resulting embryo implants in the thickened uterus lining, then progesterone levels stay high instead of falling. This keeps the uterus lining in place to support the developing embryo, and the high progesterone also stops further eggs maturing, so menstruation does not happen and the cycle pauses during pregnancy. If the egg is not fertilised, progesterone and oestrogen fall, the lining can no longer be maintained, and it breaks down as menstruation — and the cycle begins again. Understanding this helps explain why hormonal contraceptives based on progesterone are so effective: by keeping progesterone-like signals high, they trick the body into behaving as though an egg does not need to be released.

Negative feedback in the menstrual cycle

The menstrual cycle is another example of the negative-feedback control you met with thyroxine, blood glucose, temperature and water balance — the same idea running through the whole of B3. The clue is in the words inhibits and stimulates in the hormone table.

• When oestrogen rises, it inhibits FSH. Less FSH means no further eggs mature, so the ovary does not release egg after egg — the rise in one hormone switches another off.
• After ovulation, progesterone rises and inhibits both FSH and LH. This keeps the cycle in its second, "maintaining" phase and prevents a new egg maturing while the body waits to see whether the released egg is fertilised.
• If there is no fertilisation, progesterone falls. Because the thing inhibiting FSH has gone, FSH can rise again, a new egg begins to mature, and the whole cycle restarts.

So each hormone's rise triggers responses that eventually bring about its own fall, and that fall allows the next stage to begin — the classic self-correcting shape of negative feedback, here spread over about 28 days rather than seconds or minutes. Recognising this connects reproduction back to the big idea of the topic and is exactly the kind of synoptic link that lifts an answer.

Exam Tip: If a question asks why oestrogen or progesterone inhibits FSH, the mark-earning idea is that this stops further eggs maturing — a feedback control that keeps the cycle orderly. This same inhibition is what the contraceptive pill exploits.

Worked Example — Using the hormones to explain the pill

The combined contraceptive pill contains oestrogen and progesterone. Using your knowledge of the menstrual-cycle hormones, explain how taking it prevents pregnancy.

Step 1 — recall the key control link. Both oestrogen and progesterone inhibit FSH.

Step 2 — apply it to the pill. Because the pill keeps oestrogen and progesterone levels high, FSH is kept inhibited all the time.

Step 3 — work out the consequence. With FSH inhibited, no egg matures, so no egg is released at ovulation — and if no egg is released, it cannot be fertilised.