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Fibroids are lumps that grow in your womb. They're not cancer and they don't usually cause any problems. We're not sure why women get fibroids, but they are common.

If you have heavy periods, painful periods or a feeling of fullness or discomfort in your pelvis, you may want to get checked by your doctor to see if you have fibroids.

If you have fibroids and they're causing problems, there are many treatments that can help. The best treatments involve surgery. But if you're nearing menopause (when your periods stop), you may choose to wait. Fibroids often get better on their own around this time.

• Fibroids are very common and are usually harmless.
• As many as 3 in 4 women may have fibroids. But less than half of these women have any symptoms.
• The most common symptoms are heavy periods, painful periods or a feeling of fullness or discomfort in your pelvis.
• Fibroids are easy to diagnose. Doctors usually diagnose them with an ultrasound scan.
• Fibroids don't usually keep you from getting pregnant or make your pregnancy difficult.

To understand what happens if you have fibroids, it helps to know more about your womb.

Your womb has three layers.

• The inner layer is the lining (or endometrium).
• In the middle is a thick layer of muscle called the myometrium.
• The outside has a thin cover called the serosal layer.

Every month, the lining of your womb grows thicker to get ready for pregnancy. This is controlled by two hormones called estrogen and progesterone.

If you don't get pregnant, the lining of your womb falls away and out of your vagina. This is your monthly period.

The changes in your womb lining are part of your menstrual cycle. This is the monthly set of events that causes an egg to come out of your ovaries. It also causes your period to happen if you aren't pregnant.

To learn more, see What happens every month.

Fibroids are lumps that grow in your womb. They are made of the same cells that form certain kinds of muscles. Doctors sometimes call them tumors, but fibroids aren't cancer and don't turn into cancer.

You can have just one fibroid or many. The average number is six.¹ They can be tiny, about the size of a pinhead. Or they can grow to be large, sometimes as big as a balloon.² The average size is two-thirds of an inch.¹ They usually grow slowly.

They can grow:³

• On the inside of your womb, just under the lining
• In the layer of muscle.
• On the outside of your womb, just under the cover.

But very often fibroids are what doctors call "mixed." This means they grow into two different parts of your womb.

Doctors don't know what causes fibroids. The hormone estrogen seems to make them grow.⁴

When you go through menopause, your body stops making so much estrogen. So your fibroids start to shrink. If you have a treatment called hormone replacement therapy (HRT for short), your fibroids may start growing again.⁵

We don't know why some women get fibroids and others don't. But we do know that some things make women more likely to get fibroids. These are called risk factors.

Two things that give you a higher chance of having fibroids are having no children and being obese. If you are obese it means you are very overweight.

African-American women are at higher risk of getting fibroids.

To read more, see Risk factors for fibroids.