Uterine fibroids are benign tumors of the smooth muscles of the uterus. They are generally asymptomatic, but commonly cause painful or heavy menstruation, pain during sex, or lower back pain. More rarely, they grow large enough to press against the bladder, causing frequent urination. A fibroid may make it difficult to get pregnant, but this is rare. Mutiple fibroids are common. They most commonly affect women during their child bearing years and tend to shrink after menopause. Estimates vary, but 20-80% of women may develop a fibroid by the age of 50. They are generally round and white or tan in color.