Fibroids are non-cancerous growths that develop in or around the womb (uterus).
The growths are made up of muscle and fibrous tissue, and vary in size. They're sometimes known as uterine myomas or leiomyomas.
Many women are unaware they have fibroids because they don't have any symptoms.
Women who do have symptoms (around 1 in 3) may experience:
In rare cases, further complications caused by fibroids can affect pregnancy or cause infertility.
As fibroids don't often cause symptoms, they're sometimes diagnosed by chance during a routine gynaecological examination, test or scan.
See your GP if you have persistent symptoms of fibroids so they can investigate possible causes.
If your GP thinks you may have fibroids, they'll usually refer you for an ultrasound scan to confirm the diagnosis.
Read more about diagnosing fibroids.
The exact cause of fibroids is unknown, but they have been linked to the hormone oestrogen.
Oestrogen is the female reproductive hormone produced by the ovaries (the female reproductive organs).
Fibroids usually develop during a woman's reproductive years (from around the age of 16 to 50) when oestrogen levels are at their highest.
They tend to shrink when oestrogen levels are low, such as after the menopause when a woman's monthly periods stop.
Fibroids are common, with around 1 in 3 women developing them at some point in their life. They most often occur in women aged 30 to 50.
Fibroids are thought to develop more frequently in women of African-Caribbean origin.
It's also thought they occur more often in overweight or obese women because being overweight increases the level of oestrogen in the body.
Women who have had children have a lower risk of developing fibroids, and the risk decreases further the more children you have.
Fibroids can grow anywhere in the womb and vary in size considerably. Some can be the size of a pea, whereas others can be the size of a melon.
The main types of fibroids are:
In some cases, subserosal or submucosal fibroids are attached to the womb with a narrow stalk of tissue. These are known as pedunculated fibroids.
Fibroids don't need to be treated if they aren't causing symptoms. Over time, they'll often shrink and disappear without treatment, particularly after the menopause.
If you do have symptoms caused by fibroids, medication to help relieve the symptoms will usually be recommended first.
There are also medications available to help shrink fibroids. If these prove ineffective, surgery or other, less invasive procedures may be recommended.
Read more about treating fibroids.