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.
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