Overview

Cycle lengthMean ≈28 days; normal 24–38

Menstrual loss≈30–40 mL typical

Window of implantation≈days 20–24 (LH + 6–10 days)

Menstrual phase (≈days 1–5): withdrawal of progesterone (and estrogen) at the end of the luteal phase increases endometrial prostaglandins (PGF2α), endothelin and matrix metalloproteinases. Spiral arteries constrict intermittently, the functional layer becomes ischaemic and necrotic and is shed with blood; local fibrinolysis prevents clotting. Normal blood loss is ≈30–40 mL (heavy if it interferes with quality of life; historically >80 mL).

Proliferative phase (≈days 5–14, coinciding with the late follicular phase): rising estradiol regenerates the functional layer from the basalis — epithelium re-covers the surface, glands lengthen (straight, then slightly coiled), stroma proliferates, spiral arteries grow, and progesterone receptors are induced. On ultrasound the endometrium shows a "triple-line" pattern.

Secretory phase (≈days 15–28, coinciding with the luteal phase): progesterone stops proliferation and transforms the estrogen-primed endometrium — glands become coiled with glycogen-rich secretions, stroma becomes oedematous and then pre-decidual, spiral arteries coil. The window of implantation is ≈days 20–24. Without hCG, luteolysis triggers the next menstruation.

Location: Endometrium of the uterine body and fundus.

Parts & subdivisions

Menstrual phase
Shedding of the stratum functionalis after progesterone withdrawal.
Proliferative phase
Estrogen-driven regeneration; corresponds to the follicular phase.
Secretory phase
Progesterone-driven secretory and pre-decidual change; corresponds to the luteal phase.

Function

Prepares a receptive endometrium for each potential pregnancy.
Removes the non-receptive endometrium when pregnancy does not occur.

Physiology: Normal parameters (FIGO): cycle frequency 24–38 days, duration of bleeding ≤8 days, cycle-to-cycle variation ≤7–9 days. Menarche typically at ≈12–13 years; early cycles are often anovulatory.

Clinical significance

Dysmenorrhoea

Primary dysmenorrhoea results from excess prostaglandin-induced myometrial contraction and ischaemia; NSAIDs and hormonal contraception are effective. Secondary dysmenorrhoea suggests endometriosis, adenomyosis, fibroids or PID.

Abnormal uterine bleeding

Causes classified by FIGO as PALM-COEIN: Polyp, Adenomyosis, Leiomyoma, Malignancy/hyperplasia; Coagulopathy (e.g. von Willebrand disease), Ovulatory dysfunction, Endometrial, Iatrogenic, Not yet classified.

Amenorrhoea

Primary (no menses by 15 with secondary sexual characteristics) or secondary (none for ≥3–6 months). Pregnancy first; then hypothalamic, pituitary (prolactin), thyroid, ovarian (PCOS, POI) and outflow causes (Asherman syndrome, imperforate hymen, Müllerian agenesis).

Test yourself

Which phase of the menstrual cycle has the most constant length?

Menstrual phase
Follicular (proliferative) phase
Luteal (secretory) phase
All are equally variable

Answer: Luteal (secretory) phase

The corpus luteum has a fairly fixed lifespan, so the luteal phase is ≈14 days. Differences in cycle length mostly reflect variation in the follicular phase — so ovulation occurs ≈14 days before the next period, not necessarily on day 14.