The lowdown: This disorder occurs when the tissue that normally forms the lining of your uterus — the
endometrium — appears outside your uterus, usually ending up on your ovaries, bowels or somewhere else in your pelvis, says Taraneh Shirazian, MD, an ob - gyn at the Icahn School of Medicine at Mount Sinai in
New York City.
By the end of the cycle the
endometrium (the mucous membrane in the uterus that prepares the uterus for pregnancy) is used up, causing the initiation of the
new menstrual cycle.