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During your menstrual cycle, the lining of your womb (called the endometrium) thickens, as it gets ready for a fertilised egg to implant and develop into a pregnancy. If you don't become pregnant, you shed this lining out through your vagina. This is your period.

Endometriosis is when the cells of the endometrium also grow outside your womb, most often in your pelvis (pelvic lining, ovaries, bowel, ligaments). Occasionally it grows in other parts of your body.

The cells outside your womb respond to the hormones estrogen and progesterone, just as the ones inside your womb do. They bleed and break down in the same way as the lining of your womb. This causes inflammation in your pelvis, and over time scar tissue can form.

If the cells grow on your ovaries, they can sometimes form cysts (fluid-filled sacs) called endometriomas.

Endometriosis is not an infection, is not contagious, and is not cancer.

Who gets endometriosis?

Endometriosis is very common. It affects at least one in 10 women (usually in their reproductive years) of all ethnicities and social backgrounds. Sometimes endometriosis runs in families.

What causes endometriosis?

We don't know the exact cause of endometriosis, but there are several theories. The most likely is called retrograde menstruation. This means that during your periods, some blood flows back through your fallopian tubes, into your pelvis, carrying endometrial cells with it. The cells then implant in your pelvis and continue to grow.

What are the symptoms?

The most common symptoms of endometriosis are pelvic pain, painful periods, and pain during or after sex. They usually happen around the time of your period – often starting a couple of days before your period and going away in the second or third day of your period. Some women have pain when they defecate (poo).

However, some women have no pain at all, and only discover they have endometriosis when they have difficulty getting pregnant.

Endometriosis can cause problems with getting pregnant, but this doesn't happen to everyone. So if you have endometriosis and don't want to get pregnant you still need to use contraception.

How is it diagnosed?

It can be difficult to diagnose endometriosis, as often the symptoms are the same as having bad period pain. Even irritable bowel syndrome, pelvic inflammatory disease, or bladder inflammation can cause similar symptoms in some people.

The only way to see if it is definitely endometriosis is to have a laparoscopy (keyhole surgery). This involves a surgeon inserting a small tube, which has a camera and instruments attached, into your abdomen, through your belly button. The surgeon can then take pictures of what is happening, and also take tissue samples. If they find endometriosis, the surgeon will often cut or burn it out at the same time.

Endometriosis can't be diagnosed by ultrasound or swabs, but your GP may suggest doing these to check if an infection (especially chlamydia or gonorrhoea ) or another problem (like a cyst on one of your ovaries) is causing your pain.

Your doctor may also prescribe hormone treatment (such as the Pill, mini pill, or a Mirena IUD). They may also prescribe pain relief, to see if this helps with the pain. It will help if you keep a pain diary, to monitor how bad the pain is, when it happens, and if it's connected to your periods.

If you have responded well to treatment with hormones and pain relief you probably won't need surgery.

On the next page: How is endometriosis treated?

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Written by GP liaison, Christchurch Women's Hospital. Adapted by HealthInfo clinical advisers. Last reviewed November 2016.

See also:

Pelvic Pain

Period Problems

Preparing for your doctor's visit

Understanding your vaginal swab results

Page reference: 32552

Review key: HIEND-30280