Detailed definition
The endometrium is a hormonally responsive mucosal lining composed of glandular epithelium and stroma overlying the myometrium. It is divided into a basal layer (preserved through menstruation) and a functional layer (cyclically shed). During the proliferative (follicular) phase of the menstrual cycle, estrogen drives endometrial thickening from approximately 1 mm to 6–10 mm. After ovulation, progesterone from the corpus luteum produces secretory differentiation, with glycogen-rich glandular secretions and stromal predecidualization. If pregnancy does not occur, the corpus luteum regresses, hormone support withdraws, and the functional layer sheds as menstruation. After menopause, the endometrium atrophies in the absence of cyclic hormone stimulation; postmenopausal endometrial thickness on ultrasound should typically be ≤4–5 mm. Unopposed estrogen in a woman with a uterus produces continuous proliferation, simple and complex hyperplasia, atypical hyperplasia, and ultimately endometrioid endometrial carcinoma. Progesterone or progestin protects against this progression.
Why it matters in menopause
Endometrial protection is the central HRT-specific reason women with a uterus need progesterone. Postmenopausal bleeding on HRT requires evaluation — typically transvaginal ultrasound and often endometrial biopsy — because it can signal hyperplasia or carcinoma.