Endometriosis happens when tissues that normally lines the inside of your uterus or known as endometrium grows outside of the uterus especially around the pelvic area, bowel and in rare instances in the stomach or lungs.
It is a known fact that Endometriosis is an estrogen dependent disorder that can result in substantial morbidity, including pelvic pain, multiple operations and infertility (source: The Lancet).