The dark color of menstrual flow is often caused by the following clinical reasons: 1. Abnormal position of the uterus, such as a forward-flexed uterus or a backward-flexed uterus. In this case, the menstrual blood cannot be discharged outside the uterus in time and accumulates in the uterus, which turns into old bleeding when it is eliminated, so the color is darker. The actual fact is that you can get a lot more than just a few of these.