What are the causes of enlarged uterus?

Multiple things: An enlarged uterus can be caused by pregnancy, fibroids (benign muscular tumors of the uterus) or by adenomyosis which is where tissue from the lining of the uterus grows into the muscle wall causing it to enlarge, become "boggy", tender and may bleed more heavily. There is also a rare cancer, leiomyosarcoma, that can cause rapid enlargement of the uterus.