Causes of excessive menstrual flow include endometriosis, endometrial hyperplasia, and placement of intrauterine devices. 1. Endometriosis: Endometriosis can cause an increase in menstrual flow, as well as prolonged menstrual periods, probably because the ectopic endometrium also has a secretory function, which together with the normal endometrium causes the symptoms. 2. Endometrial hyperplasia: endometrial hyperplasia leads to thickening of the endometrium, which may affect the hemostatic function when the endometrium is shed, resulting in increased bleeding and prolonged menstrual period. 3. Placement of intrauterine device (IUD): Placement of IUD may lead to a transient increase in menstruation, but it will return to normal in a short period of time, which is a normal phenomenon. There are many reasons for excessive menstruation, you should go to the hospital in time, complete the relevant examinations to clarify the cause of the disease, and treat the symptoms under the guidance of the doctor.