Endometriosis is a painful disorder in which tissue that normally lines the inside of the uterus (endometrium) grows outside of the uterus. It most commonly involves the ovaries, fallopian tubes, and the tissue lining the pelvis. Rarely it will spread beyond pelvic organs. This tissue will act as it normally would while in the uterus, it will thicken, break down, and bleed with each menstrual cycle, but this displaced tissue has no exit and will become trapped. When the ovaries are involved cysts may form. Eventually the surrounding tissue will become irritated, developing scar tissue and adhesions. Fertility problems may develop if left untreated.
Causes included retrograde menstruation, transformation of peritoneal cells, embryonic cell transformation, surgical scarring especially from a C-section or hysterectomy, or immune system disorders.
Symptoms involve painful periods (dysmenorrhea), excessive bleeding, infertility, pain with intercourse, and pain bowel movements or urination. The severity of pain is not necessarily a reliable indicator of the extent of the condition.
Endometriosis is sometimes mistaken for other conditions that cause pelvic pain such as pelvic inflammatory disease or ovarian cysts. It may also be confused with irritable bowel syndrome, which can be comorbid with endometriosis, complicating the diagnosis.
Endometriosis. Mayo Clinic. https://www.mayoclinic.org/diseases-conditions/endometriosis/symptoms-causes/syc-20354656