Menstrual blood is dark red in color, and in addition to blood, there are fragments of endometrium, cervical mucus and shed vaginal epithelial cells. Menstrual blood contains prostaglandins and a large amount of fibrinolytic enzymes from the endometrium. The reasons for the dark reddish-black color include physiological factors and the presence of infection in the uterine cavity, which need to be analyzed specifically. 1. Physiological factors: when the patient is stimulated by cold and other factors, such as cold during menstruation, such as rain, freezing, cold water hand washing, etc., will lead to poor discharge of menstrual blood, so that the color of menstrual blood appears dark red and black. 2. Infection in the uterine cavity: when there is infection in the uterus, if there is endometritis, or even the presence of uterine adhesions, etc., there will also be dark red and black blood, which often has a small amount of menstrual flow. 3. Endocrine disorders: If there are disorders of estrogen and progesterone secretion in the body, such as ovulation disorders, and insufficient progesterone in the second half of the cycle, which affects the shedding of the endometrium, dark red and black menstrual blood may also occur. There are many other reasons why menstrual blood may be dark red and black, so it is recommended that patients go to the hospital in time to get a clear picture of the cause of the disease and get early treatment to avoid delaying their condition.