Endometriosis is not well understood and is probably multifactorial in origin. The most widely embraced theory involves retrograde menstruation. Although reflux of menstrual fluid occurs in many, if not all, women, in endometriosis the refluxed cells implant in the pelvis, bleed in response to cyclic hormonal stimulation and increase in size along with progression of symptoms.
Immune alterations may also contribute to the persistence of implants or endometriosis-associated infertility. Two other theories have received support. One holds that peritoneal epithelium can be "transformed" into endometrial tissue, perhaps because of chronic inflammation or chemical irritation from refluxed menstrual blood.
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