Adenomyosis is a condition in which the endometrium (uterine lining), grows into the muscle of the uterine wall where it would not normally be found, and can result in the trapping of blood and tissue inside the uterus, thereby preventing the normal sloughing process of menstruation from effectively taking place. This condition can be difficult to diagnose without proper testing, and may be confused with other conditions, such as endometriosis (the growth of the uterine lining outside of the uterus) or uterine fibroids (non-cancerous uterine tumors).Occasionally, the endometrium can grow abnormally, and when this happens, the result is usually one of two conditions, endometriosis or adenomyosis. While in endometriosis, uterine tissue actually grows outside of the uterus, in adenomyosis, the uterine tissue remains inside the uterus, but it swells and protrudes from its normal position into the uterine muscle, sometimes creating areas of trapped fluid and tissue. Adenomyosis most often occurs in women in their late reproductive years, although it is not limited to this group.