There are several possibilities for the menstrual blood to become black in the late menstrual period: first, the normal situation, because with the gradual cleanliness of the menstruation, there are residual in the uterine cavity is slowly discharged in the late period, menstrual blood in the body for a little longer will lead to black color and less amount. Second, you need to consider gynecological inflammation, such as endometrial inflammation, pelvic inflammatory disease, etc. Women who have fibroids or intrauterine device will also have black menstrual blood near the end of menstruation. Third, women with unstable ovarian function and abnormal hormone levels may also appear. Fourth, overwork and poor rest during menstruation will increase the symptoms of pelvic congestion, which will also have an effect on the color of menstruation. It is recommended to take a good rest during menstruation, and if the black menstrual blood lasts for a long time, it is recommended to have an ultrasound examination.