Besides pregnancy, there are several causes of an enlarged uterus. Two of the most common causes are uterine fibroids and adenomyosis. Uterine fibroids are common, non-cancerous tumors of the muscular wall of the uterus, affecting as many as eight in 10 women by the age of 50. The uterus sometimes may increase to double or triple its normal size with adenomyosis. It usually occurs after women have had children, and it disappears after menopause. Adenomyosis is a normally benign, but often painful condition where endometrial tissue, which normally lines the uterus, exists inside the uterus and grows into it’s the muscular walls.