The most common sites for endometriosis include:

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

The cause of endometriosis is unknown. However, a number of theories may help explain the growth of endometrial cells outside the uterus.

Theories include:

• Endometrial cells may spread from the uterus, through the fallopian tubes, and into the pelvis, where they attach to the surface of other organs.
• The lymphatic system may carry endometrial cells to other structures inside the pelvis.
• Cells that line other organs inside the pelvis transform into endometrial cells.