Images credit: US National Institutes of Health.
The wall of the uterus has two layers of tissue. The inner layer, or lining, is the endometrium. It is to this inner, mucous membrane to which the egg attaches after fertilization. The outer layer of the uterus is muscle tissue called the myometrium.
In women of childbearing age, the lining of the uterus grows and thickens each month to prepare for pregnancy. If a woman does not become pregnant, the thick, bloody lining flows out of the body through the vagina. This flow is called menstruation.