How tissue can reach the pelvis
One leading explanation begins with retrograde menstruation: during a period, some menstrual fluid flows backward through the fallopian tubes into the pelvic cavity instead of leaving the body. It may carry cells from the uterine lining with it. This backward flow is common, but endometriosis is not—so it cannot, by itself, explain why the condition develops in some people and not others.
Researchers think other factors may help determine whether those cells survive and form lesions, including how the immune system responds and how local tissue reacts. The precise causes are not fully understood. In some cases, endometriosis can also appear in places connected with prior surgery, suggesting that cells may sometimes be moved by other routes.
Why the location matters
Once established, these lesions can respond to hormonal changes across the menstrual cycle. Nearby tissue may become inflamed, and repeated irritation can contribute to scar-like bands called adhesions, which may bind organs together or restrict their movement. Pain can result from this local inflammation and scarring—not simply from the amount of bleeding.
That helps explain why symptoms vary: a small lesion can cause substantial pain if it affects a sensitive area, while the extent of visible disease does not neatly predict how someone feels. The tissue’s location can shape both the symptoms and the structures affected.
