What Causes Endo?

Remember, this is educational information only and not medical advice (and I am not a doctor nor an expert on endo). Always check with your qualified medical professional before making any changes to your treatment plan.


No one knows the cause of endometriosis, but evidence is stacked against the theory of retrograde menstruation. Also known as Sampson’s theory, this theory is that menstrual blood leaks out of the tubes into the pelvis and these bits of endometrium implant themselves in the pelvis where they land and grow into endometriosis. This 100-year-old theory has never been proven and in fact, has been debunked as the cause of endometriosis. I did an in depth Instagram post about some of the evidence against retrograde menstruation (with sources) here. The most damaging aspect of retrograde menstruation is that many ineffective treatment options are based on this unproven and incorrect theory, at a detrimental cost to us patients.

While there are several theories to the cause of endometriosis – and there may indeed be multiple factors and causes playing into why a person has it – one theory of origin is Mulleriosis. This proposes that endometriosis is due to a developmental defect in the formation of the Mullerian duct system when we are embryos (with involvement of the HOX genes). The Mullerian duct is the very beginnings of the reproductive system that will later form the fallopian tubes, uterus, uterine cervix, and superior aspect of the vagina. Cells that are laid down during its formation may have abnormal migration (they may not go to the right spot) or abnormal differentiation (when a cell changes from one cell type to another) and later develop into endometriosis.

In fact, in one study endometriosis was found during autopsy in fetuses assigned female at birth (who died for other reasons) at around the same rate, 10%, that it is found in the adult population of people assigned female at birth today. The theory of embryonic origin can explain:

  • how endometriosis can be found anywhere in the body.

  • how endometriosis has been reported in the medical literature in around 20 people assigned male at birth. This could be because as embryos, we have the beginnings of both the (what’s considered) “male” and “female” reproductive tracts during the first 6-8 weeks in the womb. Typically, one regresses and the other develops, so that the embryo has that specific set of reproductive organs. However, cells from the reproductive track that regressed can remain behind.

  • why endometriosis is found in embryonic patterns in the body and has low recurrence after true excision.

“The definitive cause(s) of endometriosis remain under debate, though demonstrated association with a number of hereditary, environmental, epigenetic and even certain menstrual characteristics exist. Current research implicates HOX genes, mesenchymal stem cells and certain immunologic factors in disease origin; nonetheless, no single theory explains endometriosis all those affected; more likely, a composite of several mechanisms is involved.” [Quote from the Center for Endometriosis article: Endometriosis: a Complex Disease]

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