There are many reasons for the increase in size of the uterus, and the common causes are as follows. First, uterine fibroids. Fibroids in any part of the uterus can increase the size of the uterus. Multiple fibroids can increase the size of the uterus significantly and in an irregular shape. When fibroids degenerate, they may increase in size suddenly or be accompanied by pain in the lower abdomen. Second, adenomyosis. Adenomyosis is caused by the growth of the endometrium into the middle of the myometrium, which increases the size of the uterus. The enlargement of the uterus is more obvious during menstruation and is accompanied by severe menstrual pain. On examination, the uterus is often hard in texture and less mobile, in addition to being enlarged. Third, hypertrophy of the uterus, also known as chronic myometritis of the uterus, the enlarged uterus is hard and often has a history of multiple births or multiple abortions. Fourth, functional uterine bleeding disorder. Pre-menopausal functional uterine bleeding will often make the uterus enlarged, but this enlargement is not the increase is particularly obvious, the uterus is uniformly enlarged.