The endometrium is the mucous membrane that lines the uterus. During the menstrual cycle, this lining thickens, allowing the possibility of a successful implantation of an embryo. Endometriosis is a condition that occurs when the cells that line the endometrium of the womb or uterus lining are found somewhere else in the body. Common sights are the pelvic peritoneum, ovaries, bladder and bowels. Some women with this condition experience mild discomfort, while many suffer from extreme pain, heavy periods and infertility. Endometriosis is a long-term condition that affects millions of women worldwide. It is commonly diagnosed in females age 25 to 40 years and rarely occurs after menopause.