The menstrual cycle

Phases

1. 1

   Follicular (ovary) and proliferative (endometrium), day 1 to ovulation

   FSH recruits a cohort of antral follicles; the dominant follicle makes more oestradiol, which thickens the endometrium and, at low levels, suppresses FSH (negative feedback) so the rest of the cohort regresses.

2. 2

   Ovulation, around day 14 of a 28-day cycle

   Sustained high oestradiol switches the hypothalamic-pituitary response to positive feedback: the LH surge, ovulation about 36 hours after its onset. The luteal phase length is fixed at about 14 days; cycle length varies in the follicular phase.

3. 3

   Luteal (ovary) and secretory (endometrium)

   The corpus luteum makes progesterone (and oestradiol), converting the endometrium to a secretory, receptive state and raising basal temperature. Without hCG from an implanting embryo the corpus luteum regresses after about 14 days.

4. 4

   Menstruation

   Progesterone withdrawal triggers spiral artery vasoconstriction, prostaglandin release and shedding of the functional layer. Prostaglandins are why NSAIDs treat dysmenorrhoea.

Open the menstrual cycle lab

Where the cycle meets the clinic

• Anovulation means no progesterone: unopposed oestrogen, an irregular unpredictable bleed, and long-term endometrial risk. That is PCOS, perimenopause and hypothalamic states.
• A mid-luteal (day 21 of 28) progesterone confirms ovulation in fertility work-up.
• Combined hormonal contraception works by holding negative feedback: no FSH rise, no dominant follicle, no LH surge.
• Emergency contraception (levonorgestrel, ulipristal) delays or inhibits the LH surge; it does nothing after ovulation, which is why the copper IUD is the most effective option.
• Menopause is the exhaustion of the follicle pool: oestradiol falls, FSH rises without feedback.