We talk about endometriosis when the mucous membrane surrounding the uterus is found outside of it. This may occur in the fallopian tubes, ovaries or pelvic wall lining tissues as well. Most commonly, it affects the peritoneum and causes great pain. It may often be the most common cause of painful menstruation.
If mucous membranes appear elsewhere, on one hand, they cause sterile inflammation, and on the other, clusters develop around them causing chronic pain. And even if they appear at the fallopian tubes, infertility can easily develop.