Illustration of 3 types of uterine fibroids. - at the top, the intramural fibroid which is the most common. It develops in the muscle layer of the wall of the uterus. - on the left, the subserosal fibroid which develops outside the uterus in a peduncle attached to the wall of the uterus. - in the middle, the submucosal fibroid is rarer and develops within the wall of the uterus and can cause significant bleeding.