The menstrual cycle

Phase 1

Follicular phase

Days 0–10 · Key hormone: FSH

• FSH (from anterior pituitary) stimulates ovarian follicles to grow

• Multiple follicles begin developing; only one becomes dominant

• Growing follicle secretes oestrogen, which thickens the endometrium

• Oestrogen temporarily positive-feeds FSH (boosts it briefly)

• As oestrogen rises further, it negative-feeds FSH — stopping new follicles

Phase 2

Pre-ovulation

Days 11–13 · Key hormone: Oestrogen → LH

• Oestrogen continues rising to its peak (around day 13)

• Peak oestrogen switches its effect on the pituitary — now positive-feeds LH

• This causes a massive LH surge (the LH spike)

• The body is now primed for ovulation on day 14

Phase 3

Ovulation

Day 14 · Key hormone: LH

• LH spike causes the dominant follicle to rupture and release a mature oocyte (egg)

• The egg travels into the fallopian tube — viable for 24–36 hours

• LH also transforms the ruptured follicle into the corpus luteum (a temporary gland)

Phase 4

Luteal phase

Days 15–28 · Key hormones: Progesterone & Oestrogen

• Corpus luteum secretes progesterone (which also boosts oestrogen)

• Both hormones maintain the thickened endometrium for potential implantation

• Negative feedback on pituitary → FSH and LH suppressed (no new follicles)

• Days 26–28: if no fertilisation, corpus luteum degenerates

• Progesterone and oestrogen drop → endometrium sheds = menstruation

• Hormone drop removes negative feedback → FSH rises again → new cycle begins