If the color of menstrual blood is black, most of the menstrual blood is clinically caused by poor flow of menstrual blood. When the uterus is in a forward-flexed or backward-flexed position, the angle between the uterine body and the cervix is formed, and it is difficult for menstrual blood to flow out. When the menstrual blood accumulates in the uterine cavity and is then excluded from the cavity, it becomes old bleeding, so the color becomes black. The second clinical condition is also seen in cervical fibroids, large polyps in the cervix, or incomplete adhesions in the cervix, all of which may lead to poor menstrual blood flow and cause black menstrual blood, and a clinical examination is recommended to make a preliminary diagnosis.