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Ovarian and uterine cycles and their hormones

Organisms (Continuity and change) · Reproduction · note 3 of 11

Spec D3.1.5Download all Reproduction notes (PDF)Download all notes (PDF)20 pages

Ovarian and uterine cycles and their hormonesSpec D3.1.5

In short

The menstrual cycle is the ovarian and uterine cycles together, controlled by FSH and LH from the pituitary and oestradiol and progesterone from the ovary. FSH stimulates follicle growth; rising oestradiol thickens the endometrium and, by positive feedback, causes the LH surge that triggers ovulation. Progesterone from the corpus luteum maintains the endometrium and inhibits FSH and LH.

The ovarian cycle (follicle growth, ovulation, corpus luteum) and the uterine cycle (endometrium builds up, is maintained, then breaks down) together make the menstrual cycle, roughly 28 days long.

The four hormones
HormoneSecreted byMain effects
FSH (follicle-stimulating hormone)Anterior pituitaryStimulates development of follicles; stimulates follicle cells to secrete oestradiol
LH (luteinizing hormone)Anterior pituitarySurge triggers ovulation; stimulates the empty follicle to become the corpus luteum
OestradiolFollicle cells in the ovaryThickens the endometrium; low levels inhibit FSH and LH; high levels stimulate the LH and FSH surge
ProgesteroneCorpus luteumMaintains and further thickens the endometrium; inhibits FSH and LH secretion
  1. Days 1–5: the endometrium is shed (menstruation). Progesterone and oestradiol are low, so FSH secretion rises.
  2. FSH stimulates several follicles to develop; usually one matures. Its cells secrete increasing amounts of oestradiol.
  3. Oestradiol stimulates the endometrium to thicken again. At first it inhibits FSH (negative feedback).
  4. By about day 12–13 oestradiol is high. High oestradiol stimulates the pituitary to release much more LH and FSH: positive feedback causes the LH surge.
  5. The LH surge triggers ovulation at about day 14: the mature follicle releases a secondary oocyte into the oviduct.
  6. LH stimulates the empty follicle to develop into the corpus luteum, which secretes progesterone (and some oestradiol).
  7. Progesterone maintains the thick endometrium and, with oestradiol, inhibits FSH and LH (negative feedback), so no new follicles develop.
  8. If there is no pregnancy, the corpus luteum degenerates after about 10–12 days. Progesterone falls, the endometrium breaks down (menstruation) and FSH rises again: the cycle repeats.
Four stacked panels over days 0 to 28 with ovulation at day 14: FSH and LH from the anterior pituitary, with the LH surge just before ovulation; follicle development, ovulation and the corpus luteum in the ovary; oestradiol peaking just before ovulation and progesterone high in days 18 to 24; and the endometrium shed, thickening, maintained and breaking down. (opens full size in a new tab)
The menstrual cycle. High oestradiol causes the LH surge by positive feedback; progesterone from the corpus luteum inhibits FSH and LH by negative feedback.
Common mistake:

Oestradiol has both effects: low to moderate levels inhibit FSH and LH, but sustained high levels before ovulation stimulate the LH surge. Say which level you mean.

Written and checked against the IB Biology HL specification · Updated October 2026

Frequently asked questions

What is the difference between sexual and asexual reproduction?

Asexual reproduction uses one parent and produces genetically identical offspring, while sexual reproduction fuses gametes to give offspring new gene combinations. Asexual reproduction suits a parent already adapted to an existing environment. Sexual reproduction produces the variation needed for a population to adapt when the environment changes, because meiosis and fertilization reshuffle alleles.

How do hormones control the menstrual cycle?

FSH from the pituitary stimulates a follicle to grow and secrete oestradiol, which thickens the endometrium. High oestradiol causes an LH surge by positive feedback, triggering ovulation. The corpus luteum then secretes progesterone, which maintains the endometrium and inhibits FSH and LH by negative feedback. When progesterone falls, menstruation begins.

Why are sperm small and eggs large?

Sperm are small because the male gamete has to travel to the female gamete, so it carries very little cytoplasm or food reserves. The egg does not move, so it can be large and store food for the early embryo. As a result, males produce far more gametes than females.

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