Adenomyosis is a medical condition that affects the uterus, specifically the endometrial lining. It is characterized by the presence of endometrial tissue, which normally lines the inside of the uterus, growing into the muscular wall of the uterus (myometrium). This abnormal growth can cause the uterus to become enlarged, thickened, and tender. The exact cause of adenomyosis is unknown, but there are several theories. One theory suggests that it may occur due to the invasion of endometrial cells into the myometrium during childbirth or other uterine procedures. Another theory proposes that hormonal imbalances, such as high levels of estrogen, may contribute to the development of adenomyosis.