Uterine fibroids are benign (not cancer) growths that develop from the muscle tissue of the uterus. They also are called leiomyomas or myomas. The size, shape, and location of fibroids can vary greatly. They may be inside the uterus, on its outer surface or within its wall, or attached to it by a stem-like structure. A woman may have only one fibroid or many of varying sizes. A fibroid may remain very small for a long time and suddenly grow rapidly, or grow slowly over a number of years.