The cause of endometriosis is not known.
Several theories have been suggested, including:
- genetics – the condition tends to run in families, and affects people of certain ethnic groups more than others
- metaplasia – it has been proposed that normal tissues outside the uterus can be replaced by endometrial (lining of the womb) tissue
- retrograde menstruation – when some of the womb lining flows up through the fallopian tubes and embeds itself on the organs of the pelvis, rather than leaving the body as a period
- a problem with the immune system, the body's natural defence against illness and infection
- endometrium cells spreading through the body in the bloodstream or lymphatic system (a series of tubes and glands that form part of the immune system) and can reach any area of the body including the brain, lungs and eyes
But none of these theories fully explain why endometriosis occurs. It's likely the condition is caused by a combination of different factors.
Content supplied by the NHS and adapted for Ireland by the HSE