Menstruation that keeps coming for 20 days and the endometrium is 10 millimeters thick is certainly not normal because the normal duration of menstruation for women is 2-8 days. Menstruation comes for 20 days without stopping and the endometrium is 10 millimeters thick, which may be caused by ectopic pregnancy, anovulatory abnormal uterine bleeding, and endometrial lesions, etc. 1. Ectopic pregnancy. Women who are sexually active and have this kind of menstrual cycle that is always late and the endometrium is thick, need to take a blood test for HCG first to determine whether they are pregnant or not. If you are pregnant and you can’t see a gestational sac in the uterine cavity, you may have an ectopic pregnancy. If it is an ectopic pregnancy, you can inject drugs such as methotrexate, or you can have surgery. 2. Anovulatory abnormal uterine bleeding. Adolescent or perimenopausal women may experience this abnormal bleeding because their ovaries do not ovulate. You can take dextroprogesterone or progesterone under doctor’s supervision to promote the shedding of the uterine lining. If there is anemia, drugs such as norethindrone can be taken, and diagnostic curettage can be performed if necessary. 3. Endometrial lesions. In the early stage of endometrial cancer, it is also easy to have this kind of long duration of menstruation and thick endometrium. If endometrial cancer is identified, early surgical treatment is generally recommended.