Endometriosis is a disease that affects the female reproductive system. It’s caused by the endomentrium (the tissue that normally lines the uterus) growing outside the uterus. According to endometriosis.org, though the cause of the disease is unknown, researchers are getting close to figuring out what the possible triggers are.

Here are six possible causes of endometriosis:

1. Metaplasia
When normal tissue changes to abnormal tissue, metaplasia happens. There are times when the endometrial tissue can, in some cases, replace other types of tissues outside the uterus.

2. Retrograde menstruation
In the 1920s, Dr. John Sampson believed that endometriosis could be caused by menstrual tissue flowing in reverse through the fallopian tubes and landing on the pelvic organs where it may stay and grow. Researchers later found that even though one in nine women have retrograde flow, not all of them develop endometriosis. Physicians now believe something else may be triggering the disease.

3. Genetic disposition 
There is a genetic link in first-degree relatives, so family history could be one of the reasons women develop endometriosis.

MORE: Did you know that many celebrities have endometriosis?

4. Lymphatic or vascular distribution
Doctors believe that some endometrial fragments may travel through blood vessels or the lymphatic system to other parts of the body.

5. Immune system dysfunction
Having a weak or dysfunctional immune system may lead to endometriosis. Some women suffering from the disease show immunologic defects or dysfunctions. This may, or may not, be one of the causes.

6. Environmental influences
Environmental factors may trigger endometriosis. Several studies point to toxins as contributing to the development of the disease. These toxins can have an effect on the immune system’s response and reproductive hormones.

MORE: Four treatment options for endometriosis you might want to know about.

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