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Endometriosis

Endometriosis

What causes endometriosis?

Photo of two women outdoors

In women with endometriosis, the kind of tissue that lines the womb also grows outside of the womb or in the wall of the womb. It is not clear why this happens. There are various theories about what causes endometriosis. Several factors probably play a role.

The inside walls of the womb (uterus) are completely lined with mucous membranes known as the endometrium. These are different to other mucous membranes in the body, particularly in their ability to change: Every month, new cells grow and the tissue thickens in case a fertilized egg settles in it. The thickened tissue can then provide the egg with everything it needs in order to develop. If no egg is fertilized, most of the extra tissue that has built up is shed and leaves the woman's body during her period. The process of building up and shedding the lining of the womb is regulated by the female sex estrogen and progesterone.

This anatomical illustration of the female abdomen shows the womb from the front, without endometriosis.

Where might endometriosis tissue grow?

In endometriosis, the kind of tissue that normally lines the womb also grows elsewhere. Medically speaking, there are different types of endometriosis, depending on where the areas of endometriosis tissue (endometrial implants) are:

  • In the peritoneum (common): Here the endometriosis tissue is attached to the lining of the abdominal cavity.
  • In the muscles of the womb (common): The tissue grows into the muscular wall of the womb. The medical term for this is adenomyosis.
  • In the lower part of the pelvis (common): Here the endometriosis tissue grows in the ovaries, fallopian tubes and/or the “Pouch of Douglas” (found between the womb and rectum at the end of the bowel).
  • Outside this area of the pelvis (rare): For instance, in the bladder or bowel, or very rarely in parts of the body that are further away, like the lungs.

Every month during the menstrual cycle, the endometriosis tissue is also built up and shed outside of the womb. But because the blood and shed tissue can't leave the woman’s body through her vagina, they stay near the endometriosis tissue. Here they may lead to inflammations, which in turn can cause scarring and adhesions (when tissue sticks together).

This anatomical illustration of the female abdomen, seen from the side, shows where endometriosis tissue might grow in the abdominal cavity.

What causes endometriosis?

There are various theories about this. According to one of them, cells from the lining of the womb get into other parts of the body and settle there. It is also thought that a hormonal imbalance or a problem with the immune system plays a role. Our usually makes sure that cells from a particular organ don't grow elsewhere in the body.

The estrogen and progesterone aren't regulated properly in women with endometriosis, either: Estrogen in particular causes endometriosis tissue to grow, whereas progesterone stops it from growing. But in endometriosis, this effect of progesterone is inhibited.

According to other theories, certain cells outside of the womb can turn into endometriosis cells for no known reason. Endometriosis seems to be more common in certain families, so genetic factors might play a role too.

Deutsche Gesellschaft für Gynäkologie und Geburtshilfe (DGGG). Diagnostik und Therapie der Endometriose (S2k-Leitlinie, in Überarbeitung). AWMF-Registernr.: 015-045. 2020.

Horne AW, Missmer SA. Pathophysiology, diagnosis, and management of endometriosis. BMJ 2022; 379: e070750.

IQWiG health information is written with the aim of helping people understand the advantages and disadvantages of the main treatment options and health care services.

Because IQWiG is a German institute, some of the information provided here is specific to the German health care system. The suitability of any of the described options in an individual case can be determined by talking to a doctor. informedhealth.org can provide support for talks with doctors and other medical professionals, but cannot replace them. We do not offer individual consultations.

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What causes endometriosis?

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Updated on March 25, 2025

Next planned update: 2028

Publisher:

Institute for Quality and Efficiency in Health Care (IQWiG, Germany)

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