Endometriosis occurs when endometrial tissue, which typically grows inside the uterus, grows outside of it.
Called endometrial implants, this tissue can grow in a variety of places, including the outside of your uterus or on your ovaries. It can also form in the space between your uterus and rectum, called the pouch of Douglas. If endometrial implants grow on your fallopian tubes, they can interfere with fertility.
During your menstruation cycle, the endometrial tissue in your uterus is shed through your period. The tissue outside the uterus also prepares to shed, but because it has nowhere to go, it can’t leave the body.
In some women, this tissue is naturally broken down and absorbed; in others it forms adhesions, becomes inflamed, or leads to the development of cysts, causing the pain and discomfort associated with endometriosis.
Although the cause is unclear, endometriosis is most likely the result of a malfunction in the immune system.