The inside of the uterus has two layers. The thin inner layer is called the endometrium. The thick outer layer is the myometrium (myo = muscle) (figure 1). In women who menstruate, the endometrium thickens every month in preparation for pregnancy. If the woman does not become pregnant, the endometrial lining is shed during the menstrual period. After menopause, the lining normally stops growing and shedding.
Under normal circumstances, a woman's uterus sheds a limited amount of blood during each menstrual period (less than 5 tablespoons or 80 mL). Bleeding that occurs between menstrual periods or excessive menstrual bleeding is considered to be abnormal uterine bleeding. Once a woman enters menopause and the menstrual cycles have ended, any bleeding, other than the small amount that may occur in women on hormone therapy, is considered abnormal.
Abnormal uterine bleeding can be caused by many different conditions. This topic review discusses the possible causes of abnormal bleeding, how it is evaluated, and various treatment strategies that may be recommended.