What causes endometriosis?

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In endometriosis, the kind of tissue that lines the womb (endometrial tissue) also grows outside 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 endometrial 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 to grow. If fertilization doesn't take place and the woman doesn't become pregnant, most of the thick membrane tissue which has built up is shed and leaves her 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.

Illustration: Womb without endometriosis, seen from the front

Womb without endometriosis, seen from the front

Illustration: Effect of hormones on the lining of the womb (uterus) during a menstrual cycle

Effect of hormones on the lining of the womb (uterus) during a menstrual cycle

In endometriosis, the kind of tissue that normally lines the womb also grows in other parts of the body. Medically speaking, there are different types of endometriosis, depending on where the endometrial tissue grows.

  • In the muscles of the womb or in the wall of a fallopian tube (common): Here the tissue is attached to the lining of the womb.
  • In the lesser pelvis (common): The endometrial tissue occurs in the ovaries, fallopian tubes and the “Pouch of Douglas” found between the womb and at the end of the bowel.
  • Outside this area of the pelvis (rare): for instance, in the bladder or bowel, and very rarely in parts of the body that are further away, like the lungs.

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

Illustration: Endometrial implants in the abdomen – as described in the article

Endometrial implants (Cross-section of the abdomen)

Theories about causes of endometriosis

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

According to other theories, certain cells outside of the womb can turn into endometrial 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). AWMF-Registernr.: 015-045. 2020.

Hickey M, Ballard K, Farquhar C. Endometriosis. BMJ 2014; 348: g1752.

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Updated on March 24, 2021
Next planned update: 2024

Authors/Publishers:

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

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