The dark red color of menstrual blood is mainly caused by the high blood volume, which is seen in normal physiological conditions, as well as reproductive system diseases, endocrine system diseases, and even other system diseases. Normal physiological conditions, mainly due to the endometrium shedding of blood vessels bleeding more, or after strenuous exercise led to uterine contraction, the effect of hemostasis is not good and lead to. Disease state with fibroids patients, due to the presence of fibroids, so it will lead to the fibroids of bleeding as well as uterine contraction is poor, there will be an increase in blood appears to be dark red. Endometrial tumors can also lead to changes in the blood, the endocrine system is mainly the increase in the level of progesterone, resulting in endometrial hyperplasia too thick, resulting in an increase in bleeding at the time of shedding, the menstrual blood dark red may occur.