The color of the blood on the first and last days of menstruation is black, mainly because the menstrual flow is low on these two days. After the menstrual blood is decomposed and oxidized by bacteria in the vagina, it turns black, earthy brown or dark coffee-like changes. As long as the patient has no obvious discomfort of lower abdominal cramping and no symptoms of constant vaginal bleeding, it is perfectly acceptable to observe for the time being. Menstruation is the periodic peeling and bleeding of the endometrium that accompanies the cyclical changes of ovarian hormones. Its main component, 95%, is the venous blood in the normal human body. The normal menstrual cycle is 21-35 days, while the period is 3-7 days.