Menstrual Cycle Hormones IGCSE Biology

The Menstrual Cycle and Hormones: Oestrogen and Progesterone

Biology editorial review is supported by the named review panel. Reviewer participation may vary by guide. Checked against Cambridge IGCSE Biology 0610 (2026–2028); last reviewed August 2026.

Key concept

The menstrual cycle lasts approximately 28 days. Oestrogen causes the uterus lining to thicken and stimulates ovulation around day 14. Progesterone maintains the thick uterus lining. If no fertilisation occurs, progesterone levels drop and the lining breaks down (menstruation).

The menstrual cycle is a Supplement-only topic that requires understanding of hormone interactions. Students must describe what each hormone does and when, linking hormone levels to changes in the uterus lining and the timing of ovulation.

Overview of the 28-day cycle

The menstrual cycle is a repeating sequence of events controlled by hormones. Day 1 is the start of menstruation, when the uterus lining breaks down and is lost as a blood flow lasting about 5 days. After menstruation, the lining begins to rebuild. Around day 14, ovulation occurs: a mature egg is released from the ovary into the oviduct.

If the egg is not fertilised, the cycle resets and menstruation begins again around day 28. If fertilisation and implantation occur, the cycle is interrupted and the lining is maintained throughout pregnancy.

The role of oestrogen

Oestrogen is produced by the ovaries. Its main roles in the menstrual cycle are to stimulate the repair and thickening of the uterus lining after menstruation, and to trigger ovulation when oestrogen levels peak around day 13-14.

Oestrogen levels are low during menstruation and rise steadily during the first half of the cycle. After ovulation, oestrogen levels decrease. At Supplement level, students should note that oestrogen also stimulates the pituitary gland to release LH (luteinising hormone), which directly triggers ovulation.

The role of progesterone

Progesterone is produced by the corpus luteum (the remains of the follicle after ovulation). Its main role is to maintain the thick lining of the uterus in the second half of the cycle, preparing it for possible implantation of an embryo.

If no fertilisation occurs, the corpus luteum breaks down, progesterone levels fall, and the uterus lining can no longer be maintained. It breaks down and menstruation begins. If fertilisation occurs, the placenta takes over progesterone production to maintain the lining throughout pregnancy.

Linking hormone levels to uterus lining changes

Students are often asked to interpret a graph showing hormone levels alongside a diagram of the uterus lining thickness. The key connections are:

Event	Approximate day	Hormone change
Menstruation	Days 1-5	Low oestrogen and progesterone
Lining rebuilds	Days 5-13	Rising oestrogen
Ovulation	Day 14	Peak oestrogen triggers LH surge
Lining maintained	Days 15-28	Rising then falling progesterone
Lining breaks down	Day 28/1	Progesterone drops

When progesterone drops, the lining can no longer be sustained and menstruation restarts the cycle.

Try a focused question (3 marks)

Explain the role of progesterone in the menstrual cycle and what happens when progesterone levels fall.

Show the answer and marking guidance

Progesterone maintains the thick lining of the uterus after ovulation.
It is produced by the corpus luteum.
When progesterone levels fall (because the corpus luteum breaks down), the uterus lining breaks down and menstruation occurs.

Original Cambridge-style practice written for this site. It is not an official Cambridge past-paper question.

Common mix-ups to avoid

Check these points against your own answer before moving on.

Confusing oestrogen and progesterone roles: oestrogen thickens the lining, progesterone maintains it.
Saying ovulation occurs on day 1 (it occurs around day 14).
Forgetting that the drop in progesterone causes menstruation.
Describing the cycle without reference to specific hormones.