The uterus (womb) is the organ in which an early embryo implants and develops until birth. The body of the uterus is known as the corpus and the opening of the uterus into the vagina is known as the cervix. The thick walled uterus is structured of smooth muscle (the myometrium). The endometrium is the lining of the uterine corpus and its thickness and structure vary with the stages of the menstrual cycle. The endometrium is comprised of the stratum functionale, which is lost and subsequently replaced during each menstrual cycle, and the stratum basale which is retained after the menstrual period and which supplies cells for the renewal of the endometrium. Uterine glands are found deep within the stratum basale and it is cells from these glands that provide for repopulation of the stratum functionale. The endometrium is well supplied with blood vessels. Sloughed stratum functionale causes blood to be released from ruptured vessels and the mix of sloughed cells and blood comprises menstrual fluid.

Ordinarily, tissues remain in place throughout life. Sometimes, however, endometrial cells wander away from the uterine corpus and establish colonies elsewhere. This pathological condition is known as endometriosis.