Uterine fibroids are tumors that grow from the wall of the uterus. The wall of the uterus is made of muscle tissue, so a fibroid is a tumor made of muscle tissue. The fibroids start off very small, actually from one cell, and generally grow slowly over years before they cause any problems. Most fibroids are benign; malignant fibroids are rare. The cause of fibroids is unknown, although it is known that fibroids have a tendency to run in families. Fibroids are very common, with an estimated 50% of women having them. Fibroids can be diagnosed by pelvic examination or by ultrasound. Fibroids do not have to be removed unless they are causing symptoms such as heavy periods, irregular bleeding, or severe cramps with periods.