Adenomyosis (ad-uh-no-my-O-sis) occurs when the tissue that normally lines the uterus (endometrial tissue) grows into the muscular wall of the uterus. The displaced tissue continues to act normally — thickening, breaking down and bleeding — during each menstrual cycle.
https://www.mayoclinic.org/diseases-conditions/adenomyosis/symptoms-causes/syc-20369138

