Uterine fibroids, which your doctor may call leiomyomas or myomas, are muscular tumors that can grow on your uterus. Uterine fibroids are noncancerous growths of the uterus that often appear during childbearing years. CAUSE: Hormones;Estrogen and progesterone, two hormones that stimulate development of the uterine lining during each menstrual cycle in preparation for pregnancy, appear to promote the growth of fibroids. Fibroids contain more estrogen and progesterone receptors than typical uterine muscle cells do. Fibroids tend to shrink after menopause due to a decrease in hormone production.