Endometriosis is a common condition, where cells that typically line the uterus grow in other areas of the body.  Every month, a woman's ovaries produce hormones that tell the cells lining the uterus (womb) to swell and get thicker. The body is able to expel these cells through the process of menstruation.  If these cells (called endometrial cells) implant and grow outside the uterus, endometriosis results. The growths are called endometrial tissue implants. Women with endometriosis typically have tissue implants on the ovaries, bowel, rectum, bladder, and on the lining of the pelvic area. They can occur in other areas of the body, too.  Unlike the endometrial cells found in the uterus, the tissue implants outside the uterus stay in place during menstruation. They sometimes bleed a little bit. They grow again during the next period. This ongoing process leads to pain and other symptoms. The cause of endometriosis is unknown. One theory is that the endometrial cells shed during menstruation and travel backwards through the fallopian tubes into the pelvis, where they implant and grow. This is called retrograde menstruation. This backward menstrual flow occurs in many women, but researchers think the immune system may be different in women with endometriosis.Endometriosis is common. Sometimes, it may run in the family. Although endometriosis is typically diagnosed between ages 25 to 35, the condition probably begins about the time that regular menstruation begins.  The classic symptoms of a women with endometriosis include cyclical pelvic pain at the time of menstruation.  The most common treatments include birth control pills, other hormonal regimens, or surgery.