Blood clots during menstruation may be due to physiological reasons, such as large amount of endometrial shedding; it may also be due to pathologic reasons, such as uterine fibroids, endometrial lesions, and so on. 1. Physiological reasons: menstrual blood consists of shed endometrium, cervical mucus, blood and vaginal epithelial cells, when the bleeding is heavy and fast, and the shedding of endometrium is too much, the phenomenon of blood clots may occur. 2. Pathological causes: (1) Uterine fibroids: If uterine fibroids are present, due to the increased area of the endometrium and the poor contraction of the myometrium, etc., they may be manifested in symptoms such as heavy menstruation, blood clots, and also prolongation of the menstrual period. (2) Endometrial lesions: If there are endometritis, endometrial polyps and other diseases, it can also be manifested as increased menstrual flow, accompanied by blood clots and other phenomena. There may be other reasons for menstrual clots, so it is recommended that you consult a doctor in time to complete the relevant examinations and receive targeted treatment under the guidance of a specialist.