Adenomyosis is a diffuse thickening of the uterus that occurs when the tissue that normally lines the uterus (endometrium) moves into its muscular outer wall. When this happens in a small area, or is localized, it is called an adenomyoma.
While the cause of adenomyosis is unknown, the condition usually occurs in women older than age 30 who have had children. It is more common in women who have had uterine surgery, including a cesarean section.