It is normal for the last few days of the period to be dark in color. A woman’s menstruation is mainly caused by exfoliation and bleeding due to luteal atrophy, decreased estrogen and progesterone levels, and loss of endometrial support from high hormone levels. After the hormone drop, the endometrium of the woman will have a rapid exfoliation, so the patient usually bleeds more within the first 2-3 days and the accompanying endometrial discharge is also more. In this case, the blood and endometrium stimulate the myometrium more strongly, so the contraction force of the myometrium is stronger and the discharged blood is bright red. 2-3 days later, most of the endometrium has been exfoliated, and the bleeding volume of the patient is significantly reduced when a small amount of endometrium is exfoliated, so the stimulation force of the myometrium is less and the contraction force of the uterus is weakened. The blood in the uterine cavity and the endometrium, which is discharged from the vagina, is relatively prolonged. At this time, the blood remains in the uterine cavity and vagina for a long time, and after oxidation and coagulation, the blood turns black.