Dark blood during physiological period is considered to be low menstrual flow, abnormal uterine position, localized endometrial abnormality and so on. 1. Low menstrual flow: when low menstrual flow occurs due to anemia, endocrine disorders, etc., the slow discharge of menstrual blood will be oxidized in the acidic environment of the vagina, and then the color will be darker, which will be manifested as black blood. 2. Abnormal uterine position: If the position of the uterus is abnormal, such as anterior flexion and anterior tilt position or posterior flexion and posterior tilt position, it will lead to the formation of an angle between the uterus and the cervix which is not conducive to the outflow of menstrual blood, and the time of menstrual discharge will be prolonged, and it will be oxidized, resulting in dark blood in physiological period. 3. Local endometrial abnormality: some diseases, such as endometrial angiogenesis abnormality, endometritis and other diseases will lead to endometrial local coagulation fibrinolysis regulation mechanism abnormality or endometrial repair mechanism abnormality, cause physiological period blood black. There may be other reasons for dark blood during physiological period, it is recommended to go to the hospital in time, improve the examination to clarify the cause of the disease, and then give targeted treatment or therapy under the guidance of the doctor.