Uterine fibroids are non-cancerous tumors that grow within the smooth muscular tissue of the uterus. In medical terms, they are called leiomyomas or myomas. The size and quantity of uterine fibroids vary. Tumors range from a single microscopic seedling to multiple growths up to eight inches or more. Large fibroids can even cause the uterus to expand to a size typical for a six- or seven-month pregnancy.