The actual cause of endometriosis is unknown. There are many theories, but none fully explain why the condition occurs.

Most scientists working in the field of endometriosis do agree, however, that it is exacerbated by oestrogen. Subsequently most of the treatments for endometriosis attempt to temper oestrogen production in a woman's body in order to relieve her of symptoms.

Several theories have become more accepted, and it is possible that a combination of these factors could be causing endometriosis to develop in some women.

Retrograde menstruation
This theory was promoted by Dr John Sampson in the 1920's. When you have a period, some of the endometrium (lining of the womb) flows backwards, out through the fallopian tubes and in to the abdomen. This tissue then implants itself on organs in the pelvis and grows. It has been suggested that most women experience some form of retrograde menstruation, but their bodies are able to clear this tissue and it does not deposit on the organs. This theory does not explain why endometriosis has developed in some women after hysterectomy, or why, in rare cases, endometriosis has been discovered in some men when they have been exposed to oestrogen through drug treatments.

Genetic predisposition
The theory is that endometriosis is passed down to new generations through the genes of family members. Some families may be more susceptible to endometriosis.

Lymphatic or circulatory spread
The theory is that endometriosis tissue particles somehow travel round the body through the lymphatic system or in the bloodstream. This could explain why it has been found in areas such as the eyes and brain.

Immune dysfunction
The theory is that for some women, their immune system is not able to fight off endometriosis. Many women with endometriosis appear to have reduced immunity to other conditions. It is not known whether this contributes to endometriosis or whether it is a result if endometriosis.

Environmental causes
The theory is that certain toxins in our environment, such as dioxin, can affect the body, the immune system and reproductive system and cause endometriosis. Research studies have shown that when animals were exposed to high levels of dioxin they developed endometriosis. This theory has not yet been proven for humans.

Metaplasia is the process where one type of cell changes or morphs in to a different kind of cell. Metaplasia usually occurs in response to inflammation and enables cells to change to their surrounding circumstances to better adapt to their environment.

In the case of endometriosis, metaplasia would explain how the endometriosis cells appear spontaneously inside the body - and how they appear in areas such as the lungs and skin. It would also explain the appearance of endometriosis cells in women with no womb - or in men who have taken hormone treatments.

During development in the womb. metaplasia allows for the development of the human body as a natural process.To explain endometriosis, some researchers believe this change from one type of cell in to an endometriosis cell happens as an embryo (developing baby in the womb), when the baby's womb (uterus) is first forming.

Others believe that some adult cells retain the ability they had as an embryo, to transform in to endometriosis cells.