Causes

The cause of adenomyosis isn't known. There have been many theories, including:

• Invasive tissue growth. Some experts believe that endometrial cells from the lining of the uterus invade the muscle that forms the uterine walls. Uterine incisions made during an operation such as a cesarean section (C-section) might promote the direct invasion of the endometrial cells into the wall of the uterus.
• Developmental origins. Other experts suspect that endometrial tissue is deposited in the uterine muscle when the uterus is first formed in the fetus.
• Uterine inflammation related to childbirth. Another theory suggests a link between adenomyosis and childbirth. Inflammation of the uterine lining during the postpartum period might cause a break in the normal boundary of cells that line the uterus.
• Stem cell origins. A recent theory proposes that bone marrow stem cells might invade the uterine muscle, causing adenomyosis.

Regardless of how adenomyosis develops, its growth depends on the body's circulating estrogen.