Endometriosis occurs when the tissues behaving like endometrium or cells lining the uterus grow in other parts of the body which causes irregular bleeding, pain and possibly infertility. The abnormal growth in tissues occurs in the pelvic area which is outside the uterus on the bowel, ovaries, bladder, rectum and the lining of the pelvis. However, the growth of the tissues may take place in other parts of the body too. Every month, a woman’s ovaries start producing hormones that are responsible for stimulating the cells of endometrium in order to multiply and prepare itself for a fertilized egg. Thus, the lining of the uterus or endometrium swells and becomes thicker.