What causes endometriosis?

Photo of two women outdoors (Digital Vision / Photodisc / Thinkstock)

In endometriosis, the kind of tissue that lines the womb (endometrial tissue) also grows outside the womb. It is still 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 hormones estrogen and progesterone.

Illustration: Womb without endometriosis - as described in the article

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

  1. In the muscles of the womb or in the wall of a fallopian tube, where the tissue is attached to the lining of the womb (common);
  2. In the lesser pelvis, including the ovaries, fallopian tubes and the “Pouch of Douglas” found between the womb and rectum at the end of the bowel (common);
  3. Outside this area of the pelvis, for instance in the bladder or bowel (rare), 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

Theories about causes of endometriosis

According to a theory known as the transplantation theory, cells from the lining of the womb get into other parts of the body and settle there. It is thought that this can happen in two ways: The cells either travel through the bloodstream, or move to the abdomen in menstrual blood that flows through the fallopian tubes. When menstrual blood flows into the fallopian tubes it is known as retrograde menstruation. But this probably happens in a lot of women – and doesn't explain why the endometrial cells only grow outside of the womb in some women, and not in others. So it is believed that other factors play a role in women who have endometriosis too, i.e. that there is a hormonal imbalance or a problem with their immune system. Our immune system usually makes sure that tissue from a particular organ doesn't grow elsewhere in the body.

According to a second theory, 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. But there is currently no evidence that it is directly inherited.