First principle: the endometrium mirrors the ovary
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- Proliferative (estrogen): thin, straight glands, active mitoses.
- Secretory (progesterone): subnuclear vacuoles appear first (the earliest histological evidence of ovulation), then luminal secretion and stromal oedema/predecidual change.
Dating logic (reasoning, not rote)
The presence of secretory change proves progesterone acted → ovulation occurred. Subnuclear vacuoles ≈ post‑ovulatory day 1–2. A purely proliferative biopsy in the luteal phase suggests anovulation.
Discriminators
| Biopsy | Inference |
|---|---|
| Secretory | ovulatory cycle |
| Proliferative when secretory expected | anovulation |
| Hyperplasia (± atypia) | prolonged unopposed estrogen 🚩 |
| Irregular shedding | luteal dysfunction |
Red flags 🚩
- Endometrial hyperplasia with atypia (EIN) on biopsy is a premalignant lesion demanding definitive management (Concept 37), not reassurance.
Pitfalls
- Over‑interpreting "dating" as precise — modern practice uses the endometrium categorically (ovulatory vs not, benign vs hyperplasia/EIN) rather than as a day‑by‑day clock.
Pearls
- 🎯 Subnuclear vacuoles = earliest sign of ovulation on histology.
- 🩺 Any postmenopausal or persistently anovulatory endometrium warrants a low threshold to sample.
Next → Why exactly does the endometrium bleed, and what does that teach about abnormal bleeding? (Concept 12).