Normally, menstrual blood is dark red, but bright red blood is often caused by the endometrium shedding when the blood sinus is open and not closed, so it will lead to a large amount of bright red blood, this situation is common in the following causes: First, there is submucosal fibroids or large interstitial fibroids that affect the contraction of the uterus, so that the endometrial area increases, so there will be bright red blood, and it is often accompanied by Second, endometrial lesions, such as endometrial cancer or endometrial atypical hyperplasia, can cause menstrual bleeding; third, blood disorders, so the condition should be examined by ultrasound, gynecological examination, routine blood tests and, if necessary, laboratory tests for sex hormones.