Endometriosis occurs when the uterine lining or endometrium grows outside the uterus, usually on the surface of pelvic organs. In very rare cases, endometrium can grow in the lungs or on other distant organs and tissues. Unlike endometrial tissue in the uterus, which builds up and sheds monthly during menstruation, this tissue growing outside the uterus builds up but can't leave the body. Eventually, endometriosis can cause bumps, nodules and scar tissue on the affected organs.

Endometriosis can occur in any woman of childbearing age, regardless of race, age or whether she has given birth. Most women who suffer from endometriosis develop the condition shortly after they begin to menstruate. More rarely, endometriosis can occur after menopause, usually if a woman takes hormone therapy. There is no link between endometriosis and endometrial cancer.

Causes. Researchers don't know what causes endometriosis. One theory is that some of the tissue shed during menstruation flows into the pelvis, where it begins to grow. There also are suspected genetic factors since women with a family history of the disorder are 10 times more likely to have it than women without such a history. Because estrogen causes the tissue to grow, researchers suspect that endometriosis may be related to a disease of the endocrine system, the system that regulates the body's hormones. The immune system also may play a role-by failing to destroy the abnormal endometrial growth.

Symptoms. The most common symptoms of endometriosis are pain and infertility; however, most painful periods are not due to endometriosis. Endometriosis-related pain can occur all month but generally worsens during menstruation. Pain can also occur during intercourse or bowel movements. Although endometriosis is associated with infertility, the actual link still isn't clear beyond the fact that the endometrial tissue may physically block the egg and sperm from uniting.

Diagnosis. Diagnosis is generally based on your symptoms, a pelvic examination and ultrasound or other imaging tests. A laparoscopy may be recommended.

Treatment. A variety of options exists to combat endometriosis.

• Pain medication. These range from over-the-counter, non-steroidal anti-inflammatories such as ibuprofen, and aspirin, to prescription narcotics.
  
  
• Hormone therapy. There are at least four types of hormonal treatments used for mild endometriosis: birth control pills, progesterone (oral or injected), danazol (a synthetic male hormone sold under the brand name Danocrine), gonadotropin-releasing hormone (GnRH) agonists, administered by injection or nasal spray (Lupron).
  
  All hormonal treatments work by regulating or changing hormone levels, thus minimizing abnormal endometrial growth and reducing the menstrual flow. Once treatment is stopped, endometriosis may return.
  
  
• Surgery. Extensive endometriosis accompanied by pain may require surgery ranging from laparoscopy to destroy the abnormal endometrial growth, to hysterectomy. If your pain is severe, your surgeon may cut the nerves in the pelvis during a procedure called a presacral neurectomy.

Prevention. There is no known way to prevent endometriosis.