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If you have endometriosis, small pieces of the lining of your womb stick to other parts of your body, such as your ovaries or your bladder. Endometriosis can be painful and you may have problems getting pregnant.

If you have endometriosis, you may get pain all the time or only sometimes. It can make you feel sick or exhausted.

But some women don't get any pain. Some women only find out they have endometriosis when they see a doctor because they haven't been able to get pregnant.

There's no cure for endometriosis. But there are good treatments that can help with the pain. And there are treatments that can help you get pregnant if you want to.^{1 2}

• Endometriosis is very common. About 5.5 million women in North America have this condition.³
• It can be harmless. You only need treatment if it's painful or if you have trouble getting pregnant.
• To find out whether you have endometriosis for sure, you'll need to have a test called a laparoscopy. A surgeon will look inside your body using a small camera.
• Treatments for endometriosis include hormones, surgery or a combination of both.
• If you have endometriosis, it's important to tell your doctor if you're trying to get pregnant. Some treatments help with pain but keep you from getting pregnant.

To understand what happens when you have endometriosis, it's useful to know something about the lining of your womb.

Your womb lies inside your pelvis. This is the area between your hips.

• The lining of your womb is called the endometrium.
• Each month, the lining grows thicker. It's part of your monthly cycle. It happens when your body starts making more of a hormone called estrogen.
• If you get pregnant, the baby grows in this thick lining. If you don't get pregnant that month, the womb lining comes away from your womb and you have your period.

To learn more about your cycle, see What happens every month.

You get endometriosis when small pieces of the lining of your womb grow on other parts of your body.^{1 2}

Usually it happens to parts of your body near your womb.³ We've described the parts that are commonly affected.

Your ovaries make eggs and hormones. If you have endometriosis in your ovaries, it may form small bags of fluid called cysts.

Your fallopian tubes carry eggs from your ovaries to your womb. These tubes are where eggs may join with sperm (fertilization).

Endometriosis may grow on the outside of your womb or on the lining of your pelvis. This lining keeps organs in your pelvis from sticking together.

Endometriosis can grow on your rectum (part of your bowel) and your bladder.

Endometriosis can also spread outside your pelvis, even as far as your brain. But this is extremely rare.

The patches of endometriosis look and work just like the lining of your womb.

The patches react to the different hormones your body makes during your monthly cycle. So each month, a hormone called estrogen makes the patches grow thicker.

And every month the patches break away and bleed, just like the lining of your womb does when you have your period.

This extra blood can't drain away quickly. Your body does slowly get rid of the blood, but it causes problems while it's inside your pelvis. The extra blood can damage the area around the patch of endometriosis. And it can stop organs from working properly. You may get scars or small bags of fluid called cysts.

If the endometriosis damages your fallopian tubes or your ovaries, they may stick together and stop working properly. If this happens, you may have difficulty getting pregnant.

We don't know for sure what causes endometriosis.

One theory is that it starts when a small amount of blood from your womb flows the wrong way during your period. Instead of flowing down to your vagina, the blood flows along your fallopian tubes, and leaks into other parts of your pelvis.^{2 4 5}

This is called retrograde menstruation. It happens to most women occasionally.⁵ But only some women go on to get endometriosis.

Very rarely, endometriosis reaches the lungs or brain. Experts don't know how this happens. Cells from the lining of your womb may travel around your body in your blood.⁴

We don't know why some women get endometriosis. But there are things that increase your chances of getting endometriosis. These are called risk factors. Your age is an important risk factor.

• The chance of getting endometriosis rises from puberty onwards and peaks at about 40.⁶
• After the age of 40, the risk goes down.^{6 7}
• You're unlikely to get endometriosis once you reach menopause. After menopause your body stops producing estrogen, the hormone that makes endometriosis grow.

To read more, see Who's at risk of endometriosis?

If you're taking the birth control pill, you're less likely to get endometriosis. And your risk stays lower for up to a year after you stop taking the pill.⁶