When sharing with my closest family members and friends the struggles I face with endometriosis, the most often question asked is how does endometriosis occur? What causes it? Sadly, I don’t have an answer for them and neither do scientists. Because the fact is, endometriosis is still not fully understood. 

At this point, the facts we know about endometriosis are these: 

• Endometriosis is endometrial-like tissue found outside of the uterus. It is not the same as the cells that line the uterus, but they are similar in appearance.  
• Endometriosis is chronic and does not have a cure. There are treatments that help to relieve pain and symptoms, but they do not cure patients.  
• Laparoscopic excision by a skilled expert in endometriosis excision is the gold standard in treatment. Not the more often used, laparoscopic ablation.  
• Endometriosis has been found on all organs, including the spleen. Intestines? Check. Bladder? Check. Diaphragm? Check. Brain? Check. Skin? Check. Nasal cavity? Check. Lungs? Check. The list goes on.  

Even though we have come a long way with our understanding of endometriosis, there is still so much we don’t know; most notably, how does endometriosis occur? In this post, I’ll discuss the six theories of origin.  

Theories of Origin

Theory 1: Retrograde Menstruation 

This theory has been around for 100 years and is severely outdated and highly unlikely. This theory, also known as Sampson’s Theory, assumes that endometriosis occurs due to retrograde menstruation.  

Retrograde menstruation means that during a person’s period, some of the blood and tissue travels out of the fallopian tubes and into the pelvic cavity (1). This theory has a lot of old evidence to support it and more recent evidence that proves it’s not accurate.  

The fact remains that almost all menstruators have some retrograde menstruation at some point (3). Which leaves us still questioning why some people develop endometriosis and others don’t. Unfortunately, this theory is the most common theory taught to and believed by physicians, leading to the improper treatment of endometriosis. 

Theory 2: Induction Theory 

Hormones cause cells outside of the uterus to transform into endometrial cells (2). This theory essentially suggests that the cells in the abdomen and pelvic cavity mutate, or change, when introduced to certain hormones.  

Theory 3: Embryonic Cell Transformation

This theory suggests that endometrial-like cells may have been present since embryo development (i.e. in the womb). This theory is widely accepted as the most likely origin of endo by endometriosis experts and can also be known as the Theory of Mulleriosis. The Mullerian-duct is present in all embryos and it develops into the female reproductive system in females. In males, the Mullerian ducts get smaller to prevent the female organs from developing. 

The theory of Mulleriosis comes from the fact that endometrial-like tissue has been found in female fetuses. It suggests that when puberty strikes, hormones cause a change in these embryonic cells (2).  

This theory also supports why endometriosis can be found in anyone, without regards to gender. Yup, that means that it’s been found in males and the LGBTQ community.  

Theory 4: Post-surgical Implantation 

Endometriosis may be the result of endometrial cells attaching to surgical scars; commonly from hysterectomy or cesarean section (2). However, my issue with this theory is that it does not explain why people who have not had any surgical procedure can have endometriosis.  

This also leads me to believe that the scientists who introduced this theory, do not believe endometriosis is possible before having any type of surgical procedure.  

Theory 5: Cell Transportation 

Endometriosis may occur when endometrial cells are transported through blood vessels and/or the lymphatic system (2). This theory is very comparable to the way in which cancer is known to spread. To be clear, endometriosis is NOT cancer. But it is worth mentioning that there are similarities to the battles that endo warriors and cancer patients face.  

To me, this theory seems like a possibility. Since endometriosis has been found on all organs, except the spleen, this theory cannot be completely ruled out. However, it does not help us understand why endometriosis has been found in all genders.  

Theory 6: Altered Immune System 

In a normal immune system, the immune system can tell when cells belong in certain areas of the body and when cells don’t.  This final theory suggest that the immune system is not able to identify and destroy endometrial cells that are outside of the uterus (2). But it still leaves us questioning how the cells got there in the first place. And it leads people to believe that endometriosis comes from the uterus.  

Pulling it Together: How Does Endometriosis Occur? 

In the end, we still don’t have a definite answer as to how endometriosis occurs or what causes it. The theory that most resonates with endometriosis experts is the Theory of Embryonic Cell transformation/ Mulleriosis.  

But like everything in healthcare, more research is needed. And honestly, more than research, we need more awareness and education on endometriosis, especially in medical schools, nursing schools, and physical therapy schools. Both awareness and education will help bring more interest in research and the funds needed to see some progress.