Conditions InDepth: Endometriosis

Endometrial tissue is the inner lining of the uterus. Endometriosis is when this tissue grows outside of the uterus. The most common sites outside of the uterus include:

  • Ovaries
  • Outside surface of the uterus
  • Fallopian tubes
  • Ligaments of the pelvis
  • Spaces between the rectum, bladder, and uterus
  • Bowel and appendix (less common)
  • Lung, arm, thigh, and skin far away (rare)

Endometrial tissue is part of the menstrual cycle. It swells, breaks down, and bleeds. This process is caused by hormones. The tissue outside the uterus will also react to hormones in the same way. It will cause swelling and pain in areas outside of the uterus. It can also cause problems in women who are trying to get pregnant.

Endometrial Lesions

Copyright © Nucleus Medical Media, Inc.

Endometriosis may occur at any age. It is most common between the ages of 25 and 40. The exact cause is unknown. Some possible factors include:

  • Tissue flows backward during the period. It moves up the fallopian tubes instead of out of the body. It moves from the tubes into the belly where it may cause abnormal growth.
  • The immune system does not clean out stray tissue fast enough. The tissue has time to root and grow in unusual places.
  • Tissue pass into lymph or blood stream. It can then travel to other areas of the body and grow.
  • Genes and things in the environment may also play a role.


American College of Obstetricians and Gynecologists (ACOG). Practice bulletin no. 114: management of endometriosis. Obstet Gynecol. 2010 Jul;116(1):223-36, reaffirmed 2018
Endometriosis. ACOG website. Available at: Updated January 2019. Accessed November 2, 2019.
Endometriosis. EBSCO DynaMed Plus website. Available at: . Updated August 30, 2019. Accessed November 2, 2019.
Endometriosis. US HHS Office on Women's Health website. Available at: Updated April 1, 2019. Accessed November 2, 2019.
Last reviewed September 2019 by EBSCO Medical Review Board Elliot Levine, MD
Last Updated: 11/6/2019

EBSCO Information Services is fully accredited by URAC. URAC is an independent, nonprofit health care accrediting organization dedicated to promoting health care quality through accreditation, certification and commendation.

This content is reviewed regularly and is updated when new and relevant evidence is made available. This information is neither intended nor implied to be a substitute for professional medical advice. Always seek the advice of your physician or other qualified health provider prior to starting any new treatment or with questions regarding a medical condition.

To send comments or feedback to our Editorial Team regarding the content please email us at Our Health Library Support team will respond to your email request within 2 business days.