The exact cause of endometriosis is unknown, but there are several theories about what causes it.
Retrograde menstruation is when the womb lining (endometrium) flows backwards through the fallopian tubes and into the abdomen (tummy) instead of leaving the body as a period. This tissue then embeds itself on the organs of the pelvis and grows.
It is thought that retrograde menstruation happens in most women, but many are able to clear the tissue naturally without it becoming a problem. It is possible that this is how endometriosis occurs in some women.
Retrograde menstruation is the most commonly accepted theory for endometriosis. However, it does not explain why the condition can occur in women who have had a hysterectomy.
Endometriosis is sometimes believed to be hereditary, being passed down through the genes of family members. It can affect women of every ethnicity, but is less common in women of African-Caribbean origin and more common in Asian women than in white women. This suggests that genes may play a part.
Spread through the bloodstream or lymphatic system
Although it is not known how, endometriosis cells are believed to get into the bloodstream or lymphatic system (a network of tubes, glands and organs that is part of the body’s defence against infection). This theory could explain how, in very rare cases, the cells are found in remote places such as the eyes or brain.
Problems with the immune system
It is believed that some women’s immune systems are not able to fight off endometriosis effectively. Many women with endometriosis are found to have lower immunity to other conditions. However, this may be a result of the endometriosis rather than the cause of the condition.
It is thought that endometriosis may be caused by certain toxins in the environment, such as dioxins, that affect the body, the immune system and the reproductive system.
However, while research suggests there is a link between endometriosis and high levels of dioxin exposure in animals, it is not currently known if this is also the case in humans.
Metaplasia is the process of one type of cell changing into another to adapt to its environment. It is this development that allows the human body to grow in the womb before birth.
It has been suggested that some adult cells may retain this ability to change, and that the shedding of menstrual blood or blood products into the pelvis during menstruation may stimulate them to transform into endometrial cells.