Doctors don't yet know exactly what causes endometriosis, a health problem that by some estimates affects more than 6.5 million U.S. women. It happens when the endometrium—tissue that normally lines the womb—grows where it doesn't belong.

Here are five key facts to understand about this noncancerous condition:

1. Most often, the displaced tissue affects pelvic organs. These include the ovaries, fallopian tubes and other surfaces of the womb. But endometrial tissue can also spread far away—for example, to the brain or lungs.

2. It can affect any female who's having menstrual periods. But typically it affects women in their 30s and 40s.

3. Pain is the most frequent symptom. Endometriosis may trigger:

• Very painful menstrual cramps that may get worse over time.

• Pain during or after sex.

• Long-term back pain.

• Intestinal pain.

• Painful bowel movements or pain when urinating during your period.

Other symptoms of endometriosis include bleeding between periods, digestive difficulties—including diarrhea, constipation, bloating and nausea, especially during periods—and trouble getting pregnant.

4. Treatment can help. Tell your doctor if you're having symptoms of endometriosis. A treatment plan will depend on factors such as your age, how severe your symptoms are and if you want to have children.

If you don't want to get pregnant, your doctor may advise hormonal birth control (either as a pill or shot) as a first treatment. It can ease—or stop—pain and bleeding. But it works best when pain or other symptoms aren't severe.

If you do want to conceive, your doctor may prescribe a medicine called a gonadotropin-releasing hormone agonist. This temporarily causes menopause. But it also stops the growth of endometriosis. Your periods will come back when you stop taking the drug, and it may be easier to get pregnant.

And what if hormonal treatments don't help? Then your doctor may suggest surgery to remove any patches of displaced tissue. There are different types of surgery available. So talk with your doctor about what makes sense for you.

5. Endometriosis sometimes goes away on its own. This often happens when periods stop. But remember: Until then, treatment may ease—or eliminate—symptoms.

Sources: National Institutes of Health; Office on Women's Health