Abnormal uterine bleeding can be caused by fibroids, hormonal imbalances or other certain causes, but sometimes, the cause is not known. Each month, if a woman doesn't become pregnant, the blood-rich tissue of the endometrium, or lining of the uterus, is shed in a menstrual period. The normal amount of blood varies from woman to woman, but bleeding that soaks a pad or tampon every hour, bleeding that lasts more than a week, or bleeding between periods can be signs of a problem.
