Endometriosis is a condition in which tissue that looks like the lining of the uterus develops outside of it. The endometrium is the lining of the uterus. Endometriosis is a condition in which endometrial tissue grows on the ovaries, intestines, and pelvic tissues. Endometrial tissue rarely spreads outside of the pelvic region, but it can happen. An endometrial implant is endometrial tissue that grows outside of the uterus. The displaced endometrial tissue is affected by the hormonal fluctuations of the menstrual cycle, causing it to become irritated and uncomfortable. This indicates that the tissue will expand, thicken, and eventually break down. The broken tissue eventually has nowhere to go and becomes trapped in the pelvis. Endometriosis manifests itself in a variety of ways. Some people have modest symptoms, while others have symptoms that range from moderate to severe. The intensity of the discomfort has nothing to do with the severity or stage of the condition.