Tissue where it does not belong
Endometriosis is a condition in which tissue resembling the lining of the uterus grows outside the uterus, commonly on the ovaries, the tissues holding the pelvic organs, and the surface of the pelvis. This misplaced tissue responds to the same cyclical hormone changes as the normal lining, so it builds up and breaks down with each cycle. Unlike the uterine lining, the resulting blood and debris cannot easily leave the body, which provokes inflammation in the surrounding tissue.
Several explanations are proposed, including retrograde menstruation, but the exact cause is not fully established and is likely multifactorial.
Why it causes pain and other effects
Repeated cycles of bleeding and inflammation can lead to scar tissue and adhesions that bind organs together, and this process contributes to pelvic pain, painful periods, and in some people difficulty conceiving. The severity of visible disease does not always match the severity of symptoms, which can make it hard to recognise.
This is general educational information about the condition, not medical advice; diagnosis and management of endometriosis require a qualified clinician.