What Causes Endometriosis?

Learn about the theories and known risk factors for this often painful disorder.

One of the first questions on a person’s mind when diagnosed with a health condition is: What caused this? 

Unfortunately, this question does not always have an answer, as is the case with endometriosis. 

The word endometriosis refers to both the condition itself as well as the abnormal tissue that characterizes the condition. This abnormal tissue is similar to endometrium, the tissue that lines the uterus and is shed during menstruation. While endometrium is confined to the uterus, endometriosis grows outside the uterus and on organs surrounding the uterus, such as the fallopian tubes, the ovaries, and the connective tissues in the pelvic region. 

This can result in a number of symptoms that can range in severity. Some women with endometriosis only have mild symptoms, while others experience severe pelvic pain, heavy menstruation and painful intercourse. Endometriosis can damage the reproductive organs, and some women with endometriosis may experience difficulty getting pregnant. 

What causes endometriosis is unknown, and medical research has not been able to explain why some women have endometriosis and others do not. However, there are several theories as to why endometriosis occurs. 

Retrograde menstruation 
This is one of the oldest theories as to the cause of endometriosis. It is sometimes described as “reverse menstruation.” The basic idea is that during menstruation, menstrual blood containing endometrium tissue moves out of the body, but also moves through the fallopian tubes and onto the organs surrounding the uterus. This is believed to occur in the majority of women, but in women who have endometriosis, this tissue grows and implants into other tissue. 

Circulatory or lymphatic spread 
Another theory proposes that endometrial tissue spreads to other parts of the body through the circulatory or lymphatic systems. The idea is that endometrial cells are carried through the body along with blood or lymph (a fluid containing white blood cells) and implant in other tissues. This theory would help explain why endometriosis can be found in distant areas of the body, such as the lungs, brain and skin. 

This term refers to the ability of certain cells to change into different types of cells. This theory proposes that cells located outside the uterus change into endometrial cells in response to hormonal and/or inflammatory factors. This theory could help explain why endometriosis has been found in females who have not yet experienced menstruation, why it can be found in distant parts of the body, and why it can occur in people who do not have a womb, such as women who have undergone hysterectomy and (in very rare cases) males. 

Additional theories and risk factors 
None of the theories mentioned here adequately explain what causes endometriosis and why it occurs in some people and not others, and research is ongoing. In addition to what is discussed above, there are other potential factors that may play a role in the development of endometriosis: 

  • Genetics. Females with a first-degree family member (such as a mother or a sister) who has endometriosis are at a higher risk of having endometriosis. 
  • Immune system. One role of the immune system is to destroy abnormal cells, which should mean that the immune system destroys endometriosis cells. People who have endometriosis may have a defective immune response that allows endometriosis cells to persist. 
  • Hormones. Endometriosis appears to be associated with an imbalance or abnormal functioning of the hormones estrogen and progesterone, though research is ongoing.  

Risk factors for endometriosis include experiencing first menstruation before age 11, short monthly cycles (fewer than 27 days in duration), heavy menstrual bleeding, menstrual bleeding that lasts longer than 7 days, giving birth for the first time after the age of 30 and never having given birth.  

No risk factor is a guarantee that a person will have a disease or disorder, and as mentioned above, none of these theories adequately explain what causes endometriosis. The best thing women with and without endometriosis can do for their reproductive health is work with a gynecologist for regular exams and to address any health issues and concerns. 

Medically reviewed in October 2019. Updated in February 2021. 

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