Whether or not uterine fibroids have any effect on menstruation depends on the size of the fibroids as well as the location of the growth. Submucosal fibroids, no matter how large, have an effect on menstruation, manifesting as heavy menstrual periods, prolonged periods, and dribbling. Uterine fibroids growing between the muscle walls increase in size and compress the endometrium, also affecting menstruation, showing excessive menstruation, prolonged menstruation, and dribbling. Subplasma fibroids, however, do not affect menstruation. The most important reason why fibroids affect menstruation is that they cause the uterine cavity to become larger and the shedding of the endometrium to be incomplete. So fibroids affecting the lining of the uterus can cause irregular menstruation.