The endometrium is the tissue that lines the uterus. During each menstrual cycle, a new lining grows, getting ready for a possible pregnancy. If you don't become pregnant during that cycle, the lining sheds. This is your menstrual period.

Endometriosis (say "en-doh-mee-tree-OH-sus") occurs when tissue that is similar to the endometrium grows outside of your uterus. It usually grows on the ovaries or the fallopian tubes. It also may grow on the outside surface of the uterus, the bowels, or other organs in the belly.

These growths are called "implants." They grow, bleed, and break down with each menstrual cycle, just like the lining of the uterus does. In some women, this can cause pain and can make it hard to get pregnant.

Sometimes scar tissue forms around implants. This also can cause pain and trouble getting pregnant.

The female hormone estrogen, released by the ovaries, makes the implants grow.

While some women never have symptoms, others have severe pain. In some cases, the problem can affect how well your bowels, bladder, or other organs work.

Pain from implants may be mild for a few days before your menstrual period. It may get better during your period. But if an implant grows in a sensitive area such as the rectum, it can cause constant pain or pain during sex, exercise, or bowel movements.

Symptoms often get better during pregnancy and after childbirth and usually go away after menopause.

You likely will start treatment with birth control hormones. They are usually the safest hormones for long-term use. If they don't help your symptoms, you could take a stronger hormone. And if that one doesn't work, you could try an even stronger hormone. The stronger hormones are used only for a short time, because they have serious side effects.

Hormone therapy reduces estrogen levels in your body. Because of this, you can't use hormone therapy if you want to get pregnant.

• Birth control hormones (such as a patch, pills, or a ring) stop ovulation and the growth of implants. These hormones can be used long-term until menopause.
• Gonadotropin-releasing hormone agonist (GnRH-a) therapy lowers estrogen to the levels women have after menopause. It's usually used for only 3 to 6 months. Women sometimes take it with progestin to reduce side effects. GnRH-a is given as a shot or as a nasal spray.
• Progestin lowers estrogen. It shrinks implants and reduces pain. Progestins are given as a shot or a pill. The levonorgestrel-releasing intrauterine device (LNG-IUD) is also used.
• Danazol lowers estrogen levels and raises male hormone (androgen) levels. It shrinks implants and reduces pain for most women. Because of its side effects, though, it is not chosen as often as the other hormones. And it's used for no more than 6 to 9 months.
• Aromatase inhibitors help stop the body from making estrogen. These medicines are used along with hormone treatment.

Hormones relieve the symptoms of endometriosis for many women. You may have to try two or more types of hormones before you find one that works for you.

If taking birth control hormones works for you, you can use them for years (unless you plan to get pregnant).

For some women, hormones work for a while. For others, relief lasts a long time.

After treatment with any hormone therapy, pain may come back.

These medicines have different side effects and risks. Some of them can make you feel like you're going through menopause.

• Birth control hormones. Side effects are usually mild and often go away after the first few months. They can include spotting between periods, nausea, headaches, breast tenderness, and mood changes. There is a small increase in the risk of getting blood clots. Your doctor will not prescribe these hormones if you have had blood clots before, have had breast cancer, or are older than 35 and smoke.
• Gonadotropin-releasing hormone agonist (GnRH-a). Side effects can include hot flashes, mood swings, vaginal dryness, less interest in sex, trouble sleeping, and headaches. Risks also include bone thinning.
• Progestin. Side effects may include mood changes and depression, bloating and weight gain, weight loss, breast tenderness, and absent or light and irregular periods. High-dose progestin can cause bone thinning. Talk to your doctor about whether the progestin intrauterine device (Mirena) might give you the same benefits with fewer side effects.
• Danazol. Side effects include decreased breast size, acne, and more facial and body hair. It can also cause deepening of the voice, which can be permanent. It can increase the risk of "bad" cholesterol.
• Aromatase inhibitors. They can cause headaches, nausea, diarrhea, aching joints, and hot flashes. You also have a risk of bone thinning if you use these medicines for a long time.

Your doctor may advise you to try hormones if:

• Your symptoms are mild.
• You are close to menopause. Hormones might ease your symptoms until menopause.