Pain can be the first sign that something's wrong - cramping pain in the pelvis or pain during sex. But for women of childbearing age, difficulty getting pregnant is most often what leads them to see a doctor. The culprit, endometriosis, is difficult to diagnose and may require a surgical procedure.

Endometriosis, or "endo" for short, is caused by tissue that normally grows along the lining of the uterus that begins to grow elsewhere - on the ovaries, for example. This "unauthorized" growth may be what causes pain.

Researcher Dr. Pamela Stratton is chief of the gynecology consult service at NIH's National Institute of Child Health and Human Development (NICHD). She says the really puzzling thing is that the degree of pain is not always related to the amount of endo. A woman feeling low levels of pain might have a number of large endo cell masses, yet a woman in a lot of pain might have only trace amounts.

Treatments may reduce her pain and improve fertility. The good news is that endo can often be treated while it is being diagnosed. There are two types of tests that can show images of unauthorized tissue growth - ultrasound and MRI - but only a surgical procedure called laparoscopy can confirm that the growth is endo. After masses are removed during the procedure, a lab determines if they are endo.

Ending pain permanently, however, can be more difficult, Stratton explains. For many women, the pain returns, signaling regrowth of the tissue. Because endo cells are fed by estrogen, some doctors prescribe estrogen blockers to deprive endo of its food source.

"Treating the pain by altering hormone levels gets much more tricky," Stratton says. Lowering estrogen can limit endo growth, but it can also cause unpleasant side effects like hot flashes, unwanted hair growth and weight gain.

NIH scientists are working on new ways to treat endo. In one study, researchers are hoping an estrogen drug called raloxifene can limit endo growth. Stratton and her colleagues are also working with other specialists to better understand how the brain processes the pain caused by endo. Other factors such as chronic stress may make endo pain worse.

Researchers are continuing to work on a variety of new ways to help women with endo pain.

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NIH is the nation's medical research agency - making important medical discoveries that improve health and save lives. The National Institutes of Health (NIH), a part of the U.S. Department of Health and Human Services, is the primary Federal agency for conducting and supporting medical research.