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Several theories may explain how endometriosis develops:
- One popular theory focuses on a potential process known as retrograde menstruation. Retrograde menstruation can be thought of as backward flow during a period. This is also known as the implantation theory.
- Menstrual products, including endometrial cells, may escape into the body through the Fallopian tubes and are deposited onto internal structures such as the ovaries, the bladder, and portions of the large intestine.
- These cells, once deposited, are able to respond to progesterone and estrogen in much the same way as normal endometrial tissue within the uterus.
- The growth of this misplaced endometrial tissue can cause distortion of abdominal and pelvic structures and causes the development of adhesions (scars) within the abdominal and pelvic cavities.
- Endometrial tissue can be found on the outside of the uterus, the space between the uterus and the colon known as the posterior cul-de-sac, the supporting ligaments of the uterus, the ovaries, the urinary bladder, and other internal structures.
- However, it is unlikely that retrograde menstruation alone is the cause of endometriosis, since retrograde menstruation has been shown to occur commonly in many women. Other causative factors may play roles in determining which women develop endometriosis.
- Another theory, also known as coelomic metaplasia, suggests that a layer of cells surrounding the ovaries and other cells within the pelvic region are able to change into endometrial cells that are much the same as normal endometrial tissue. It is not certain what causes this development, but evidence suggests irritation by retrograde menstrual flow or infections may be the culprit.
- Transfer of endometrial tissues by a surgical procedure might be the cause for endometriosis implants seen in surgical scars (for example, episiotomy or Cesarean section scars).
- The rare cases of endometriosis that develop in the brain or other distant organs are likely due to the spread of endometrial cells via the bloodstream or lymphatic system.
- Some studies have shown alterations in the immune response in women with endometriosis, suggesting that abnormalities in the immune system may play a role in the development of the condition.
For more information, read our full medical article on endometriosis.