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The National Institutes of Health (NIH) defines endometriosis as a condition in which the tissue that behaves like the cells lining the uterus grows in other areas of the body, causing pain, irregular bleeding, and possible infertility. The tissue growth typically occurs in the pelvic area, outside of the uterus, on the ovaries, bowel, rectum, bladder, and the delicate lining of the pelvis. However, the growths can also occur in other areas of the body.
According to the Mayo Clinic, with endometriosis, displaced endometrial tissue continues to act as it normally would: It thickens, breaks down and bleeds with each menstrual cycle. And because this displaced tissue has no way to exit the body, it becomes trapped. Surrounding tissue can become irritated, eventually developing scar tissue and adhesions.
Health care providers and researchers have said the exact cause of endometriosis is unknown. However, there are several theories.
One theory is called retrograde menstruation or reverse menstruation. The Mayo Clinic said that in retrograde menstruation, menstrual blood containing endometrial cells flows back through the fallopian tubes and into the pelvic cavity instead of out of the body. These displaced endometrial cells stick to the pelvic walls and surfaces of pelvic organs, where they grow and continue to thicken and bleed over the course of the menstrual cycle.
But because most women experience some amount of retrograde menstruation without developing endometriosis, HealthyWomen.org said, researchers believe something else may contribute to its development. For example, endometriosis could be an immune system problem or hormonal imbalance that enables the endometrial tissue to take root and grow after being pushed out of the uterus.
HealthyWomen.org went on to say that still other researchers believe in some women, certain abdominal cells mistakenly turn into endometrial cells. These same cells are the ones responsible for the growth of a woman's reproductive organs in the embryonic stage.