Endometriosis is a problem with the lining of the uterus (womb). Normally, when you have your menstrual period every month, the lining (endometrial tissue) of the uterus will come out in the menstrual flow.

Endometrial tissue is normally found only inside of the uterus. However, endometriosis refers to the presence of the endometrial tissue outside of the uterus. The most common sites include: the ovaries; the outside surface of the uterus; the fallopian tubes; ligaments of the pelvis; and the spaces between the rectum, bladder, and uterus. Less commonly the rectum, bladder, intestine, and appendix may be involved. Rarely, deposits (or implants) of endometrial tissue may be found in the lung, arm, thigh, and skin far away from the reproductive tract.

Endometrial Lesions

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This misplaced endometrial tissue responds to the monthly menstrual hormones. It swells, breaks down, and bleeds. Whereas during the normal menstrual flow the blood exits through the vagina, there is no exit when this tissue is in other locations outside of the uterus. When the blood is trapped inside the body it can irritate surrounding tissue and cause pain and scarring. The pain is increased during the menstrual cycle.

In the United States, endometriosis affects an estimated 10%-20% of women of reproductive age. Although endometriosis may occur at any age, it is most commonly seen between the ages of 25-40.

The exact cause of endometriosis is unknown. Some possible considerations include:

  • Menstrual blood and endometrial tissue flows backward through the fallopian tubes instead of forward through the vagina and out of the body.
  • The immune system does not recognize that there is foreign tissue in the body and allows the tissue to implant and develop in unusual places in the body. This causes endometriosis.
  • The lymph (glands) and/or blood systems carry endometrial cells from the uterus to other areas of the body.
  • Genetic and environmental susceptibility.