Detailed definition
The uterus is a hollow, pear-shaped, muscular organ in the female pelvis. Its three layers are the endometrium (innermost mucosal lining, cyclically responsive to estrogen and progesterone), the myometrium (smooth muscle wall responsible for menstrual cramping and labor contractions), and the perimetrium (outer serosa). The uterus has two main parts: the corpus (body) and the cervix (lower opening into the vagina). During the reproductive years, the endometrium proliferates under estrogen stimulation in the follicular phase, transforms to a secretory state under progesterone in the luteal phase, and sheds as menstruation if pregnancy does not occur. After menopause, the endometrium becomes thin and atrophic in the absence of estrogen. The uterus matters for HRT because women with an intact uterus require progestogen alongside estrogen to prevent endometrial hyperplasia and cancer.
Why it matters in menopause
Whether a woman has a uterus determines the structure of her HRT regimen. Intact uterus = combination HRT (estrogen plus progestogen). Post-hysterectomy = estrogen-only HRT. This is one of the first questions in any HRT consultation.