Endometrium and abnormal uterine bleeding

Lesion

The endometrium reads like a hormone diary. When ovulation fails or hormones are taken, the pattern changes, and that pattern explains the bleeding.

Updated Nov 12, 2026

What it is

The endometrium changes in a predictable way with estrogen and progesterone, so a biopsy can be dated and abnormal patterns recognised. Most abnormal uterine bleeding without a structural lesion comes from anovulation: estrogen stimulates the lining without the progesterone that would mature it and allow an orderly shed, so it breaks down irregularly. Structural causes (polyps, adenomyosis, leiomyomas, hyperplasia and cancer) and systemic causes (bleeding disorders, drugs) complete the picture; in postmenopausal women, atrophy is the commonest cause, but cancer must be excluded.

Structure

Menstrual endometrium: stromal breakdown with clusters of condensed stromal cells, apoptotic debris, neutrophils and blood.
Anovulatory breakdown: proliferative-type glands, some cystically dilated, with patchy stromal collapse, fibrin thrombi in small vessels and surface repair.
Luteal phase defect: secretory changes that lag behind the expected date, from too little progesterone.
Atrophic endometrium: thin lining with few small or cystic inactive glands and fibrous stroma.

Step by step

Without ovulation there is no corpus luteum, so no progesterone is made.
Estrogen keeps stimulating the endometrium, which grows thick and fragile.
Without progesterone the stroma never matures and the spiral arteries do not develop properly.
The overgrown lining outstrips its blood supply and breaks down in patches, at irregular times.
The result is unpredictable, often heavy, bleeding; long-standing unopposed estrogen also raises the risk of hyperplasia.

Causes of abnormal uterine bleeding and what the pathologist finds (PALM-COEIN groups)

Group	Cause	What the specimen shows
Structural	Polyp	Dense stroma, thick-walled vessels, dilated glands
Structural	Adenomyosis	Endometrial glands and stroma deep in the myometrium
Structural	Leiomyoma	Whorled smooth-muscle nodule, often submucosal
Structural	Hyperplasia or carcinoma	Crowded glands, with or without atypia, or invasive carcinoma
Non-structural	Coagulation disorder	Usually normal endometrium for the date
Non-structural	Ovulatory dysfunction	Disordered proliferation and anovulatory breakdown
Non-structural	Endometrial (local) cause, endometritis	Plasma cells in the stroma
Non-structural	Iatrogenic (hormones)	Inactive glands with decidualised stroma

Clinical notes

Adolescents: anovulation and inherited bleeding disorders such as von Willebrand disease are the main causes.
Reproductive years: always exclude pregnancy and its complications first.
Any bleeding after menopause needs evaluation of the endometrium by ultrasound and, when indicated, biopsy.

Test yourself

Think of the answer, then tap the question to see it.

Why does anovulation cause irregular bleeding?

Without a corpus luteum there is no progesterone, so estrogen-driven endometrium grows thick and unstable and breaks down in patches at irregular times.

What does a secretory endometrium show that a proliferative one does not?

Glandular vacuoles and luminal secretions, stromal edema and predecidual change around spiral arteries, all driven by progesterone.

What is the most common cause of postmenopausal bleeding, and what must be excluded?

Endometrial atrophy is the most common cause; endometrial hyperplasia and carcinoma must be excluded.

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Educational content on the mechanisms of disease; not a diagnostic or treatment guide and no substitute for a health professional's advice. No doses are given.

Sources: Abnormal Uterine Bleeding (StatPearls (NCBI Bookshelf), 2026).

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Representative model, for educational purposes only.