What are fibroids?

What are uterine fibroids?

Uterine fibroids (also called leiomyomas or myomas) are growths made of smooth muscle and fibrous tissue that form within the wall of the uterus. They are not cancerous. Fibroids can range in size from tiny seedlings that are undetectable by the human eye to large masses that can enlarge and distort the uterus. Some people have one fibroid; others have several.

Medical organizations including the American College of Obstetricians and Gynecologists (ACOG) and the National Institutes of Health describe fibroids as benign (noncancerous) growths that do not increase the risk of uterine cancer. Fibroids can occur in people with a uterus at any age, but they are most common during the reproductive years.

Fibroids grow in response to the hormones estrogen and progesterone, which is why they often shrink after menopause when hormone levels decline. They are distinct from cancerous tumors and from adenomyosis (a condition where uterine lining tissue grows into the uterine muscle).

How common are fibroids?

Uterine fibroids are very common. According to ACOG and the National Institutes of Health, up to 70–80% of people with a uterus will develop fibroids by age 50. Many of those people will never know they have them because they have no symptoms. Fibroids are more common among Black women, who tend to develop them at a younger age, have larger and more numerous fibroids, and experience more severe symptoms. Family history also plays a role: if a close relative has had fibroids, you are more likely to develop them.

Because many people with fibroids have no symptoms, the true number is difficult to measure precisely. Many fibroids are discovered incidentally during routine pelvic exams or imaging for unrelated reasons.

Types of fibroids

Fibroids are classified by where they grow in the uterus. The location often determines which symptoms they cause and which treatment approaches may be discussed. The main types are:

• Intramural fibroids — The most common type. They grow within the muscular wall of the uterus. When they enlarge, they can cause heavy menstrual bleeding, pelvic pressure, and pain.
• Subserosal fibroids — These grow on the outer surface of the uterus (the serosa) and can press against nearby organs such as the bladder or bowel, causing pressure symptoms, frequent urination, or constipation.
• Submucosal fibroids — These grow just beneath the inner lining of the uterus (the endometrium) and protrude into the uterine cavity. Even small ones can cause heavy or prolonged bleeding and may affect fertility or pregnancy outcomes.
• Pedunculated fibroids — These grow on a stalk (peduncle) extending from the uterus, either into the cavity (pedunculated submucosal) or outward (pedunculated subserosal).

Many people have a combination of these types. Imaging such as ultrasound or MRI can help identify the location, size, and number of fibroids and how they relate to surrounding structures.

Symptoms (and when there are none)

Many people with fibroids have no symptoms at all. For those who do, the most common symptoms include:

• Heavy or prolonged menstrual bleeding (menorrhagia)
• Painful periods (dysmenorrhea)
• Pelvic pressure or fullness
• Frequent urination or difficulty emptying the bladder
• Constipation or rectal pressure
• Lower back pain or leg pain
• An enlarged abdomen or a sensation of abdominal swelling
• Pain during intercourse
• Difficulty getting pregnant or complications during pregnancy

Not having symptoms does not mean fibroids are harmless, but it also does not mean they will become a problem. Many people with asymptomatic fibroids can be monitored without intervention. The decision to treat fibroids depends on the presence and severity of symptoms, the size and location of the fibroids, fertility goals, and overall health.

If you experience any of the above symptoms, discussing them with a clinician is the appropriate first step. Symptoms like heavy bleeding, pelvic pressure, and bladder changes can also have causes other than fibroids, so a proper evaluation is important before assuming fibroids are responsible.

How are fibroids diagnosed?

Fibroids are often first suspected during a routine pelvic exam, when a clinician feels an irregularly shaped or enlarged uterus. The most common imaging test to confirm a fibroid diagnosis is a pelvic ultrasound, which uses sound waves to create images of the uterus and identify the size, number, and location of fibroids. An MRI may be used when more detailed information is needed, such as before a planned procedure or when ultrasound results are unclear.

Other tests that may be part of the workup include:

• Complete blood count (CBC) — To check for anemia from heavy bleeding.
• Sonohysterography — An ultrasound with saline infusion to better evaluate the uterine cavity, especially for submucosal fibroids.
• Hysteroscopy — A camera inserted through the cervix to directly view the inside of the uterine cavity.
• Transvaginal ultrasound — A more detailed view using an ultrasound probe placed in the vagina.

Your clinician can recommend which tests are appropriate based on your symptoms, exam findings, and treatment goals.

Are fibroids cancer?

Fibroids are almost always noncancerous (benign). Major patient-facing medical sources describe cancer arising from a presumed fibroid as rare. Having fibroids does not mean you have cancer, and fibroids are not thought to increase the risk of uterine cancer.

Still, new or changing bleeding, rapidly worsening pain or pressure, postmenopausal bleeding, or symptoms that feel unusual for you should be discussed with a clinician. Your clinician can decide whether additional evaluation is needed based on your symptoms, exam, and imaging history.

When to seek care

If you have symptoms that may be related to fibroids, a non-urgent visit with a primary care clinician or gynecologist is an appropriate starting point. However, certain situations require more urgent or emergency attention. Seek prompt medical care for:

• Severe or uncontrolled vaginal bleeding (soaking through a pad or tampon every hour for several hours)
• Fainting, lightheadedness, or signs of significant blood loss
• Sudden severe pelvic or abdominal pain
• Fever with pelvic pain (which may suggest infection or fibroid degeneration)
• Pain or bleeding during pregnancy
• Postmenopausal bleeding (any bleeding after menopause should be evaluated)
• Chest pain, shortness of breath, or leg swelling (which may suggest a blood clot)

This list covers the most common urgent concerns but is not exhaustive. When in doubt, contact a clinician or seek urgent/emergency care.

Sources

ACOG uterine fibroids FAQ · MedlinePlus uterine fibroids · Office on Women's Health fibroids · NHS fibroids overview