endometrium

The endometrium is the mucous membrane lining the interior of the uterus. It undergoes cyclic changes in response to ovarian hormones. The endometrium is divided into two main layers: the functional layer (stratum functionalis), which thickens and sheds during the menstrual cycle, and the basal layer (stratum basalis), from which the functional layer regenerates. The tissue contains glands, stroma, and a rich blood supply from spiral arteries.

During the menstrual cycle, the endometrium thickens in the proliferative phase under estrogen, followed by the secretory phase after ovulation when progesterone promotes glandular secretion and stromal edema. If fertilization does not occur, the functional layer is shed as menses. Typically, thickness varies with cycle phase and individual physiology, ranging from a few millimeters during menstruation to more substantial thickness during the secretory phase.

In early pregnancy, the endometrium supports implantation and becomes decidualized, forming the maternal portion of the placenta. The endometrium can be affected by disorders such as endometritis (infection), endometriosis (endometrial-like tissue outside the uterus), and endometrial cancer (the most common gynecologic malignancy in some populations), with risk factors including obesity and prolonged exposure to unopposed estrogen.