Hormones in Human Reproduction

A hormone is a chemical messenger, made by a gland and carried in the blood to a target organ where it produces an effect. During puberty, reproductive hormones start to be released in much larger amounts, and they cause the body to mature so that reproduction becomes possible.

These hormones trigger the development of secondary sex characteristics — the physical features that distinguish adult males and females but are not the reproductive organs themselves.

Sex	Main reproductive hormone	Where it is produced	Examples of secondary sex characteristics it triggers
Female	Oestrogen	Ovaries	Breasts develop, hips widen, menstrual cycle begins
Male	Testosterone	Testes	Facial and body hair, deeper voice, muscle growth

Oestrogen is the main female reproductive hormone, produced in the ovary. Testosterone is the main male reproductive hormone, produced by the testes.

Both hormones are present in all bodies in small amounts; the difference is the quantity produced from puberty onward.

Beyond triggering secondary sex characteristics, the two main reproductive hormones control the production of gametes (sex cells).

In females — oestrogen, produced in the ovary, drives the changes of puberty and the monthly menstrual cycle. From puberty, eggs begin to mature inside the ovaries, and one egg is released about every 28 days. The release of an egg is called ovulation.

In males — testosterone, produced by the testes, stimulates sperm production. Unlike egg release, sperm production is continuous once it begins at puberty rather than running on a monthly cycle.

Hormone	Produced in	Main role in reproduction
Oestrogen	Ovaries	Main female hormone; controls puberty and menstrual cycle
Testosterone	Testes	Main male hormone; stimulates sperm production

Learn the pairing precisely: oestrogen → ovary, testosterone → testes. The exam frequently asks where each hormone is produced.

The menstrual cycle lasts roughly 28 days and prepares the body for a possible pregnancy each month. If the released egg is not fertilised, the thickened lining of the uterus (womb) breaks down and is lost — this is the period (menstruation).

Four hormones control the cycle. You must know each one, where it is made, and its job.

Hormone	Produced by	Role in the menstrual cycle
FSH (follicle stimulating hormone)	Pituitary gland	Causes an egg to mature in the ovary
LH (luteinising hormone)	Pituitary gland	Stimulates the release of the egg (ovulation)
Oestrogen	Ovaries	Helps build and maintain the uterus lining
Progesterone	Ovaries	Maintains the uterus lining

Two hormones come from the brain's pituitary gland (FSH and LH); two come from the ovaries (oestrogen and progesterone). Sorting them by gland is a quick way to keep the four straight under exam pressure.

A common way to remember the order of events: FSH matures the egg, then LH lets it go.

The cycle is usually described in four stages. Day 1 is the first day of the period. The timeline below tracks which hormone is responsible at each point.

Stage by stage:

1. Days 1–5 (period) — the uterus lining is shed. FSH is released and begins to mature an egg.
2. Days 6–13 — the maturing egg's follicle produces oestrogen, which thickens and builds the uterus lining ready to receive a fertilised egg.
3. Day 14 (ovulation) — a surge of LH stimulates the release of the mature egg from the ovary.
4. Days 15–28 — progesterone maintains the thick uterus lining. If the egg is not fertilised, progesterone levels fall, the lining breaks down, and the cycle restarts at day 1.

The egg is released around day 14 of a 28-day cycle — roughly the midpoint. Cycle lengths vary between individuals, so 28 days is an average, not a fixed rule.

(Higher Tier only — Foundation students need the four roles above, but do not need the detailed feedback interactions on this slide.)

The four hormones do not act independently; they control each other through the cycle. Higher Tier answers must explain these interactions.

(Higher Tier only — the four interactions to learn.)

1. FSH stimulates the ovaries to produce oestrogen.
2. Oestrogen stimulates the release of LH — and at the same time inhibits (stops) further FSH being released.
3. LH triggers ovulation (release of the egg).
4. Progesterone maintains the uterus lining and inhibits both FSH and LH.

Walk-through of the feedback: FSH from the pituitary makes the ovary produce oestrogen. As oestrogen rises, it switches off FSH (so no more eggs start to mature) and switches on LH. The LH spike causes ovulation around day 14. After the egg is released, progesterone is produced and keeps the lining thick; because progesterone inhibits FSH and LH, no new egg matures while progesterone is high. When progesterone falls (no pregnancy), that inhibition is lifted, FSH rises again, and the next cycle begins.

(Higher Tier only — quick check.) If a graph shows oestrogen peaking just before a sharp spike in another hormone, that other hormone is LH, because oestrogen stimulates LH and the LH surge causes ovulation.

(Higher Tier only — interpreting hormone-level graphs is a Higher Tier skill assessed under MS 2c.)

Exam questions often give a graph of hormone levels across the cycle and ask you to read or explain it. Use the timing of each peak to identify the hormone.

Feature on the graph	Most likely hormone	Reason
Higher early in the cycle (days 1–5)	FSH	FSH starts egg maturation at the beginning
Rises steadily up to about day 12	Oestrogen	Oestrogen builds the lining before ovulation
Sharp spike around day 14	LH	The LH surge triggers ovulation
High in the second half (days 15–28)	Progesterone	Progesterone maintains the lining after ovulation

Worked interpretation: a graph shows hormone X rising from day 6 and peaking near day 12, immediately followed by hormone Y spiking sharply at day 14. Hormone X is oestrogen (it builds the lining and rises before ovulation); hormone Y is LH (the spike that triggers ovulation). The link is the feedback rule: oestrogen stimulates LH.

When asked why ovulation happens at a certain day, name the LH surge — do not just say "the egg is released." The cause is the LH peak.

1. Mixing up where each hormone is made

FSH and LH are made by the pituitary gland in the brain; oestrogen and progesterone are made by the ovaries. Writing that FSH is made in the ovary is a frequent error.

2. Confusing the jobs of FSH and LH

FSH matures the egg; LH releases the egg. They are easy to swap. Remember: FSH comes first (maturation), LH comes second (the day-14 surge that causes ovulation).

3. Saying oestrogen causes ovulation

Oestrogen builds the uterus lining and (on Higher Tier) stimulates LH — but it is the LH surge that directly triggers ovulation. Name LH as the cause.

4. Treating 28 days as exact for everyone

The 28-day cycle and day-14 ovulation are averages. Cycle length varies between individuals, so describe these figures as approximate.

5. (Higher Tier) Getting the feedback direction wrong

Oestrogen inhibits FSH and stimulates LH; progesterone inhibits both FSH and LH. A common slip is saying progesterone stimulates these hormones — it suppresses them, which is why no new egg matures while progesterone is high.