Adenomyosis or internal endometriosis occurs when the tissue that lines the inside of the uterus penetrates the myometrium, or muscular lining of the uterus.
During menstruation, the tissue inside the myometrium swells and bleeds in the same way as the lining of the uterus. This causes blood to pool within the uterine muscle, where it can form benign tumours known as adenomyomas.