A uterine leiomyoma is a noncancerous growth made of smooth muscle and connective tissue that develops in or on the uterus. You probably know them by their more common name: fibroids. By age 50, somewhere between 69% and 83% of women will have at least one, though most will never know it because the majority cause no symptoms. About 25% to 30% of women with fibroids experience symptoms significant enough to need treatment.
How Fibroids Form and Grow
Each fibroid starts as a single smooth muscle cell in the uterine wall that begins dividing abnormally. Over time, it builds up a dense mass of muscle fibers surrounded by a thick web of collagen and other structural proteins. This combination of muscle and connective tissue is what gives fibroids their firm, rubbery texture and distinguishes them from the softer surrounding uterine tissue.
The two main reproductive hormones, estrogen and progesterone, are the primary fuel for fibroid growth. Estrogen increases the number of progesterone receptors on fibroid cells, essentially making them more responsive. Progesterone then drives the real growth: it stimulates fibroid cells to multiply, suppresses the natural process of cell death that would normally keep growth in check, and increases collagen production that adds bulk to the tumor. This hormonal dependence explains why fibroids typically develop during the reproductive years, grow during pregnancy when hormone levels surge, and shrink after menopause when hormone production drops.
Where Fibroids Grow in the Uterus
A fibroid’s location matters as much as its size when it comes to symptoms. Doctors classify fibroids into three main categories based on where they sit in the uterine wall.
- Submucosal fibroids grow into the inner cavity of the uterus, pushing against or protruding through the lining. Some hang into the cavity on a stalk. Even when small, these are the most likely to cause heavy bleeding because they interfere with the blood vessels in the uterine lining during menstruation.
- Intramural fibroids are embedded entirely within the muscular wall of the uterus. These are the most common type. They may enlarge the uterus as they grow, creating a feeling of pressure or fullness.
- Subserosal fibroids grow outward from the outer surface of the uterus. Some are mostly buried in the wall, while others extend outward on a stalk. These are less likely to cause bleeding but can press on the bladder or bowel as they enlarge.
Many women have fibroids in more than one location at the same time, and the combination determines which symptoms, if any, they experience.
Who Gets Fibroids
Fibroids are overwhelmingly common. A landmark study using ultrasound on women aged 35 to 50 in the Washington, D.C., area found fibroids in 69% to 83% of participants by the time they reached 50. Black women face a disproportionately high burden: their incidence is roughly three times higher than that of white, Asian, or Hispanic women, and fibroids tend to appear at younger ages and grow larger.
Other factors associated with higher risk include starting menstruation early, having a family history of fibroids, obesity (since fat tissue produces additional estrogen), and never having been pregnant. Women who have had children tend to develop fewer fibroids.
Common Symptoms
The majority of fibroids are discovered incidentally during a routine pelvic exam or imaging for something else. When symptoms do develop, they generally fall into a few patterns.
Heavy or prolonged menstrual bleeding is the most frequent complaint, sometimes severe enough to cause iron-deficiency anemia with fatigue, dizziness, and shortness of breath. Pelvic pressure or a sensation of fullness is common as fibroids enlarge the uterus. Some women experience painful periods, pain during sex, low back pain, frequent urination from a fibroid pressing on the bladder, or constipation from pressure on the bowel.
Symptoms can change over time. Fibroids may grow gradually for years, then undergo degeneration (where they outgrow their blood supply and begin to break down), which can cause sudden, sharp pain. They can also calcify, becoming hard and less symptomatic. The unpredictability is part of what makes fibroids frustrating to live with.
Effects on Fertility and Pregnancy
Whether a fibroid affects your ability to get pregnant depends almost entirely on where it sits. International guidelines agree on a few consistent points. Submucosal fibroids, the ones growing into the uterine cavity, are clearly linked to reduced fertility, lower implantation rates, and higher miscarriage rates. This makes sense: they physically distort the space where an embryo needs to implant.
Subserosal fibroids, growing outward from the uterus, do not appear to have a significant effect on fertility. Intramural fibroids fall in a gray area. When they’re large enough to push against and distort the uterine lining, they can reduce pregnancy rates. When the lining remains undisturbed, the impact on fertility is minimal.
During pregnancy, fibroids can increase the risk of complications including miscarriage, preterm delivery, and abnormal fetal positioning, though many women with fibroids carry pregnancies without incident.
How Fibroids Are Diagnosed
A standard pelvic ultrasound is usually the first step. Fibroids typically appear as well-defined, round masses that are easy to identify. When a fibroid grows near the inner lining, it can sometimes look similar to a polyp. In those cases, a technique called sonohysterography, where sterile saline is gently infused into the uterine cavity during the ultrasound, gives a clearer picture and helps tell the two apart.
MRI provides the most detailed view and is particularly useful before planning treatment. On MRI, fibroids appear as well-defined, round masses, often accompanied by dilated veins nearby. This helps distinguish them from adenomyosis, a different condition where tissue similar to the uterine lining grows into the muscular wall. Adenomyosis tends to appear as an ill-defined, oval-shaped area with scattered tiny bright spots representing glandular tissue, while fibroids have sharp, clean borders.
Are Fibroids Ever Cancerous?
The risk of a presumed fibroid actually being a cancerous tumor called a leiomyosarcoma is very low, estimated between 0.13% and 0.49% of women who undergo surgery for fibroids. Fibroids themselves are benign. The concern isn’t that a fibroid “turns into” cancer but rather that a leiomyosarcoma can sometimes mimic a fibroid on imaging. Rapid growth of a fibroid, particularly after menopause when fibroids should be shrinking, is one factor that raises suspicion, though no imaging test can reliably distinguish the two before surgery.
Medical Treatment Options
When fibroids cause bothersome symptoms, the first approach is often medication aimed at controlling bleeding or temporarily shrinking the fibroids.
One class of medication works by shutting down the hormonal signals from the brain that stimulate ovarian hormone production, creating a temporary low-estrogen state similar to menopause. This can shrink fibroids significantly within a few weeks but comes with side effects like hot flashes, mood changes, and bone density loss, so it’s typically used short-term, often to shrink fibroids before surgery. Hormonal add-back therapy (a small amount of hormone replacement) is usually given alongside to reduce these side effects.
Newer medications target the progesterone pathway more directly, blocking progesterone’s ability to fuel fibroid growth. These drugs have shown effectiveness in reducing fibroid size and improving symptoms like heavy bleeding. Because progesterone plays such a central role in fibroid growth, blocking its action at the cellular level addresses the problem more precisely than older approaches.
Surgical and Procedural Options
When medications aren’t enough, several procedures can remove or destroy fibroids while preserving the uterus, or the uterus can be removed entirely.
Myomectomy surgically removes fibroids while leaving the uterus intact. It can be done through open abdominal surgery, laparoscopically, or hysteroscopically (through the vagina and cervix) depending on fibroid size and location. Myomectomy has the lowest long-term re-intervention rate of the common procedures: about 19% of women need additional treatment within five years.
Uterine artery embolization is a less invasive alternative where a radiologist threads a tiny catheter through a blood vessel in the groin and injects small particles that block blood flow to the fibroids, causing them to shrink. Recovery is shorter than surgery, though about 21% of women need additional treatment within five years.
MRI-guided focused ultrasound uses concentrated sound waves to heat and destroy fibroid tissue without any incision. It’s the least invasive option but carries the highest re-intervention rate, with roughly half of women needing further treatment within five years.
Hysterectomy, removal of the uterus, is the only definitive cure and eliminates any chance of recurrence. It remains the most common surgical treatment for fibroids in women who have completed childbearing, though the trend has shifted toward uterus-preserving options as techniques have improved.