Despite the long history of endometriosis and the research happening on the condition worldwide, there is no known root cause of endometriosis.

Most theories for the causes of endometriosis include a role for the hormones that made the endometrial layer in the uterus grow during each cycle of an individual’s period. As these hormones are present in greater amounts after puberty and before menopause, endometriosis typically affects people who have a period or menstrual cycle, and it is not often seen in young individuals before they start their period or in individuals who have reached menopause.

Below are the most common theories for the causes of endometriosis:

Retrograde Menstruation

This is the most common theory for endometriosis. It states that endometriosis may be caused by a change in the direction of the flow of blood and tissue during a period or menstruation. Typically, a menstrual flow exits the uterus through the cervix and vaginal canal; in the case of retrograde menstruation, some of this flow can move backward to the fallopian tubes and exit the uterus into the pelvic cavity rather than leaving the body.

While this might sound strange and unusual, 9 out of 10 individuals who menstruate experience retrograde menstruation. This process can also occur between menstrual bleeding times in the cycle, such as shortly before ovulation. In the case of endometriosis, the blood and tissue shed during menstruation reach the pelvic and abdominal cavities and implant into the nearby organs. These patches of tissue will grow to form endometriotic growths.

Coelomic Metaplasia

Coelomic (see-lo-mic) metaplasia (meta-play-sha) states that endometriosis may be caused by tissue that is similar to the endometrial tissue in the inner lining of the uterus, developing directly on regions outside the uterus (e.g. ovaries, bladder, and bowels). It is thought that this tissue is made of cells that have a similar early origin to the endometrial cells within the uterus, typically occurring when the cells are dividing and creating a baby during pregnancy.

Under specific conditions in the individual’s life, these cells may then be triggered to develop further into endometrial tissue and create the endometriotic growths seen in endometriosis. This theory is believed to explain how endometriosis can develop in areas distant from the pelvis, such as the diaphragm and lungs.

Immunologic and Genetic Theories

Immunologic theories state that endometriosis develops from changes in the immune system.  In this theory, it is thought that the body responds to the endometrial tissue that makes up the endometrial growths as a foreign object in the body, which leads to an inflammation response to try to remove the foreign object. If the inflammation response continues for long periods of time, it can damage the nearby tissues. However, we still do not know which comes first, the endometrial tissue or the change in the immune system.  

Theories based on genetics have been proposed for endometriosis because it occurs more often in particular families. The risk of individuals with a close relative with endometriosis (e.g. a parent or sibling) also developing the condition is increased by 7 times. Though this increased risk suggests that there may be a gene responsible for endometriosis, there has currently been no specific ‘endometriosis gene’ found by scientists.

Genetic/Epigenetic Theories

The Genetic/Epigenetic Theory is the most recent theory and is based on the idea that genetic factors influence endometriosis growth. Genetics refers to the genes or DNA in your body that contain information about you, while epigenetics refers to which genes are activated depending on the influence of your environment.  Changes in the way your genes are activated can also be inherited from your parents. This theory of endometriosis, suggests that endometriotic growths can originate from endometrial tissue, bone marrow, or stem cells that have inherited genetic and/or epigenetic defects.

According to the theory, most individuals with uteruses develop really small endometriotic growths that either go away on their own or start growing into bigger problems. This theory is similar to the one used to explain the development of some tumours, however, endometriotic growths are not cancerous.

Discovery of Endometriosis

Endometriosis was first described by Daniel Schroen, a German physician, in 1690 when he looked at a growth on the skin. Later, in 1860, Karl von Rokitansky, an Austrian physician and pathologist, built upon that first description of endometriosis, allowing our knowledge of the condition to continue to grow.

Despite the long history of endometriosis and the research happening on the condition worldwide, there is no known root cause of endometriosis. This is most likely because endometriosis does not have one single cause and it often presents differently on each individual. Endometriosis can also affect many different areas of the body and symptoms vary between individuals.