The exact cause of endometriosis is not known. However, many theories exist as to how the condition occurs. Endometriosis describes a condition in which pieces of the womb lining (the endometrium) are found lying outside of the womb, usually in areas such as on the ovaries, the pelvis and towards the top of the vagina, although they may also occur in the fallopian tubes, bladder, bowel and rectum.

Causes

Hypotheses regarding the causes of endometriosis include:

Retrograde menstruation

This theory was first proposed by John A. Sampson who suggested that when a woman menstruates, some of the endometrium leaves the uterus but not the body. It may move through the fallopian tubes and attach itself to any pelvic or peritoneal surfaces instead of leaving the body through the vagina.

Most women are thought to experience some degree of retrograde menstruation but do not develop endometriosis because their bodies clear the residual endometrial tissue before it forms deposits. Genetics, toxins and a weakened immune system may play a role in increasing the risk of endometriosis in some women.

Hormonal changes

The growth of endometriosis is dependent on the female hormone estrogen. However, endometriosis has been found to persist despite low levels of estrogen, after menopause and after hysterectomy, for example.

Müllerianosis

This refers to when cells that have similar properties to endometrial cells are deposited in the female reproductive tract during embryonic development when organs are forming. The primitive endometrial cells then get dislodged form the uterus and act like stem cells.

Surgery

Bits of the endometrium may get dislodged and become embedded in an area outside of the womb during a gynaecological operation.

Other factors

Other factors that may contribute to the development of endometriosis include immune system disorders, genetic predisposition and environmental toxins.