Normally, menstrual blood is non-clotting, so it is normal to have no clots or only very small clots in the menstrual blood. If a relatively large blood clot appears during menstruation, it is clinically abnormal and is commonly due to the following causes. The first is an inflammatory reaction of the endometrium, which can lead to large clots in the menstrual blood. Secondly, submucosal fibroids or large interstitial fibroids may also cause clots in menstrual blood and may be clinically manifested as excessive menstrual flow, prolonged menstruation, or shortened menstrual cycle, which can be diagnosed by ultrasound. In addition, this condition is also seen in functional uterine bleeding, so when blood clots appear in menstrual blood, further examination must be done to clarify the cause before using medication.