We don't know the exact cause of endometriosis. Right now, a number of theories try to explain the disease.

Endometriosis may result from something called "retrograde menstrual flow", in which some of the tissue that a woman sheds during her period flows into her pelvis. While most women who get their periods have some retrograde menstrual flow, not all of these women have endometriosis. Researchers are trying to uncover what other factors might cause the tissue to grow in some women, but not in others.

Another theory about the cause of endometriosis involves genes. This disease could be inherited, or it could result from genetic errors, making some women more likely than others to develop the condition. If researchers can find a specific gene or genes related to endometriosis in some women, genetic testing might allow health care providers to detect endometriosis much earlier, or even prevent it from happening at all.

Researchers are exploring other possible causes, as well. Estrogen, a hormone involved in the female reproductive cycle, appears to promote the growth of endometriosis. Therefore, some research is looking into endometriosis as a disease of the endocrine system, the body's system of glands, hormones, and other secretions. Or, it may be that a woman's immune system does not remove the menstrual fluid in the pelvic cavity properly, or the chemicals made by areas of endometriosis may irritate or promote growth of more areas. So, other researchers are studying the role of the immune system in either stimulating, or reacting to endometriosis.

Other research focuses on determining whether environmental agents, such as exposure to man-made chemicals, cause endometriosis. Additional research is trying to understand what, if any, factors influence the course of the disease. We just don't have answers on the causes yet.

Another important area of NICHD research is the search for endometriosis markers. These markers are substances made by or in response to endometriosis that health care providers can measure in the blood or urine. If markers are found, health care providers could diagnose endometriosis by testing a woman's blood or urine, which might reduce the need for surgery.

Current estimates place the number of women with endometriosis between 2 percent and 10 percent of women of reproductive age. But, it's important to note that these are only estimates, and that such statistics can vary widely.

You are more likely to develop endometriosis if you:

• began getting your period at an early age
• have heavy periods
• have periods that last more than seven days
• have a short monthly cycle (27 days or less)
• have a close relative (mother, aunt, sister) with endometriosis

Some studies suggest that you may lower your chances of developing endometriosis if you:

• exercise regularly
• avoid alcohol and caffeine