Normally, endometrial tissue is found only inside the uterus. The uterus is the reproductive organ where a fetus develops. Hormones cause the tissue to form there, preparing the body for a fertilized egg. If you do not become pregnant, the tissue leaves the body during menstruation.

In endometriosis, endometrial-like tissue is found outside the uterus. For example, it may be found on organs in the abdomen or pelvis. In these places, the tissue still responds to hormones. It swells, breaks down, and bleeds. But it is unable to leave when you menstruate. Surrounding tissue becomes inflamed. There is often scarring.

Endometriosis
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Lesions were created by swelling and breakdown of endometrial tissue outside the uterus.
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Possible causes include:

  • Menstrual tissue backs up through the fallopian tubes and spills into the abdomen
  • Immune system may allow the tissue to implant on other organ surfaces and develop into endometriosis
  • Lymph system may carry endometrial cells from the uterus
  • Certain cells left behind on abdominal organs during embryonic development can turn into endometrial tissue

Hormones and growth factors cause the disease to progress.