Endometriosis occurs when small pieces of the lining of the womb grow elsewhere in the body - most commonly in the fallopian tubes, bladder, ovaries, bowel or vagina.
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These rogue cells follow the same pattern as those that line the womb - every month they grow during the menstrual cycle and are then shed as a bleed.
Normally before a period, the lining of the womb thickens in order to receive a fertilized egg; when the egg isn’t fertilized, the lining breaks down and leaves the body as menstrual blood. When these cells grow elsewhere in the body, though, they have no point of exit, and the tissue becomes trapped, often causing pain and swelling, generally in the pelvic area, lower back or abdomen.
Other common symptoms of endometriosis can include painful periods, pain during sex, heavy periods, bleeding from the bowel and pain during urination. In some women it can also cause infertility - there is a greater risk of this when the condition goes undiagnosed for a long time.
Although the cause of endometriosis is not known, some experts believe there is a hereditary link and are even looking into the possibility of a gene that might make people more prone to the condition. As yet, nothing has proven conclusive.