The endometrial tissue acts as a cushion whose mission is to wrap around and protect the embryo. As the cycle progresses, the endometrial tissue acquires a certain thickness and texture to facilitate embryo implantation and facilitate the initial gestation. If pregnancy has not occurred at the end of the cycle, the uterus expels the tissue in the form of bleeding.
Although the origin of endometriosis is completely unknown, it is generally accepted that its origin is from retrograde menstrual flow which, in most cases, circulates from the fallopian tubes into the pelvis causing the endometrial tissue to invade other organs or areas of the body.
This tissue acts as if it were inside the uterus, i.e., each month it will thicken due to hormonal action and undergo decomposition. However, since it is located in an area where it cannot be removed, the tissue is trapped, causing various lesions including: superficial lesions, adhesions, fibrosis (scarring), ovarian cysts (endometriomas), etc.
In endometriosis, women experience the usual hormonal changes during the cycle in the places where the tissue has entered, causing accumulation of blood, inflammation and, consequently, pain.