If there are a lot of clots in menstrual blood, it is often due to the following reasons: 1. The most common is because of submucosal fibroids, or intermural larger fibroids. As these two kinds of fibroids grow into the uterine cavity, making the endometrial area relatively larger, affecting the uterine contraction to make the blood sinus open, it will make the menstrual blood form clots. Therefore, this situation should be timely examination, through the ultrasound can be given a preliminary diagnosis. 2, because of the position of the uterus is abnormal, such as the uterus in the anterior flexion anterior tilt position or posterior flexion posterior tilt position, menstrual blood can not be discharged out of the uterine cavity, the blood pooling in the uterine cavity to form a blood clot. The color is relatively dark, and a preliminary diagnosis can be made through gynecological examination.