Normally, there is no blood clot during menstruation because menstrual blood is not clotted. If it is all blood clots, it is often seen for the following reasons: 1. Inflammation of the endometrium. Because of inflammation of the endometrium, the endometrium will shed large clots, which will lead to blood clots in menstrual blood, often accompanied by pain in the lower abdomen. 2. Organic lesions of the uterus, such as submucosal fibroids or large interstitial fibroids, can cause blood clots in menstrual blood and lead to shortened menstrual cycles, longer periods and increased menstrual blood. Therefore, a detailed examination, such as an ultrasound, is recommended to rule out fibroids and to understand the condition of the endometrium before treatment.