There are several possible causes of heavy menstrual flow due to fibroids: 1. Functional exfoliation of the endometrium and the onset of menstruation. If fibroids protrude from the uterine cavity, resulting in an increase in the area of the endometrium, the bleeding will be more than before after endometrial exfoliation; 2. The fibroids affect the contraction of the uterus, and after normal uterine contraction, the blood will stop when the blood vessels in the uterus are squeezed. Excessive menstrual flow is caused by fibroids affecting uterine contraction; 3. Infection of the organism caused by fibroids can also lead to excessive menstrual flow.