Endometriosis is a disorder in which tissue similar to the lining of the uterus grows outside the uterine cavity. The lining of the uterus is called the endometrium. The hormonal changes of the menstrual cycle affect the misplaced endometrial-like tissue, causing the area to become inflamed and painful. It means the tissue will grow, thicken, and break down. Over time, the tissue that has broken down has nowhere to go and becomes trapped in the pelvis. Women with endometriosis tend to have a lower monthly fecundity of about 0.02–0.1 per month. In addition, endometriosis is associated with a lower live birth rate.