Near the end of menstruation, the volume of menstrual flow usually decreases and the rate of menstrual blood discharge slows down, so the blood stays in the uterus and vagina for a period of time before being expelled, which is also known as stale menstrual blood and is therefore black in color. Normal menstrual blood is usually dark red in color and contains endometrial debris, cervical mucus and shed vaginal epithelium in addition to blood. Menstrual blood contains a large amount of fibrinolytic enzymes. Because of the presence of this fibrinolytic enzyme, menstrual blood is usually not clotted, and clots can occur only when bleeding is heavy or rapid. The menstrual blood observed at this time is usually dark red in color. At the end of the menstrual period, when the menstrual flow begins to decrease significantly towards the end of the period, the rate of discharge slows down at this time, and because it stays in the body for a longer period of time, the menstrual blood discharged is dark red in color.