Endometriosis is a chronic condition where tissue similar to the lining inside the uterus, known as endometrium, starts to grow outside the uterus. This can lead to significant pain and other symptoms. While it primarily affects the pelvic organs, endometriosis can involve the ovaries, fallopian tubes, and the tissue lining the pelvis. In rare cases, it can spread beyond pelvic organs.

The misplaced tissue behaves like endometrial tissue within the uterus—it thickens, breaks down, and bleeds with each menstrual cycle. However, since it has no way to exit the body, it becomes trapped and this can result in the formation of cysts, inflammation, and scar tissue.