Endometriosis occurs when the tissue that lines the inside of your uterus starts to grow outside of the uterus. The displaced tissue, called endometrial tissue or implants, adheres to other soft tissue inside your abdomen, including your ovaries, fallopian tubes, bladder, or your bowels.

Endometriosis is estrogen (hormone) dependent, meaning it requires estrogen to grow. This is why this condition usually affects women in their 20s and 30s, and often begins after only a few years of a woman starting a period.