The endometrium is the layer of cells that line the inside of the uterus. A woman with endometriosis has endometrial cells growing on the surfaces of organs that lie outside the uterus.

The most common sites include:

• Outside surface of the uterus
• Ovaries
• Bladder
• Colon
• Rectum
• Lining of the pelvis

Common symptoms include pelvic pain and heavy menstrual bleeding. About 25 percent of women of childbearing age in the US have endometriosis.