What Are Uterine Fibroids? Causes and Symptoms Explained

What Exactly Are Uterine Fibroids?

Uterine fibroids, clinically termed leiomyomata uteri, are monoclonal tumors arising from the smooth muscle cells (myometrium) of the uterine wall. They are benign. They do not become cancer in the vast majority of cases, with leiomyosarcoma occurring in fewer than 0.1% of apparent fibroid cases, according to data reviewed by the American College of Obstetricians and Gynecologists (ACOG).

Fibroid Classification by Location

Where a fibroid sits inside or around the uterus determines the symptoms it produces and the treatment options available.

• Submucosal fibroids grow just beneath the endometrial lining, protruding into the uterine cavity. Even small submucosal fibroids cause disproportionately heavy bleeding and are the type most likely to affect fertility.
• Intramural fibroids develop within the muscular wall of the uterus. They are the most common type and can enlarge the uterus significantly before symptoms appear.
• Subserosal fibroids project outward from the uterine surface toward the abdominal cavity. Large subserosal fibroids may press on the bladder or bowel rather than causing menstrual changes.
• Pedunculated fibroids attach to the uterus via a stalk. They can be intracavitary (inside the cavity) or external; pedunculated fibroids occasionally undergo torsion, causing acute pelvic pain.

A single uterus can harbor fibroids of multiple types simultaneously. A 2013 population study published in Fertility and Sterility found that among women with fibroids confirmed by ultrasound, the median number of discrete tumors was 3.

Size Variation

Fibroids range from a few millimeters to more than 20 centimeters in diameter. Clinicians often describe size by comparison: a uterus enlarged to 12 weeks of gestational size means it contains enough fibroid volume to approximate a 12-week pregnancy. Larger fibroids are not automatically more symptomatic than smaller ones; location matters more than size alone.

How Common Are Uterine Fibroids?

Fibroids are the most common benign gynecologic tumor. Cumulative incidence data from the NIEHS Uterine Fibroid Study, which used serial ultrasound in a community-based cohort, found that by age 50, fibroids were detectable in approximately 70% of white women and more than 80% of Black women. Many of these women had no symptoms.

Racial Disparities in Fibroid Burden

Black women are disproportionately affected in ways that extend beyond simple prevalence differences. A 2013 analysis in the American Journal of Obstetrics and Gynecology documented that Black women develop fibroids at younger ages (often before age 30), present with larger and more numerous tumors, and experience more severe symptoms including iron-deficiency anemia from chronic blood loss. These disparities persist after controlling for socioeconomic variables, suggesting biologic and potentially environmental contributors that are still being studied.

The disparity in surgical rates is also documented: Black women undergo hysterectomy for fibroids at roughly twice the rate of white women, even when uterus-sparing procedures are available. ACOG Practice Bulletin No. 228 acknowledges these disparities and calls for equitable access to minimally invasive options.

What Causes Uterine Fibroids?

No single cause explains fibroid development. Current evidence points to a convergence of genetic mutations, steroid hormone signaling, growth factors, and environmental exposures acting on susceptible myometrial stem cells. Research published in Nature Reviews Cancer characterizes fibroids as clonal neoplasms in which a single mutated smooth muscle cell proliferates under hormonal stimulation.

Genetic Factors

Roughly 40 to 50% of fibroids carry somatic mutations in the gene encoding mediator complex subunit 12 (MED12), a transcriptional regulator. Other recurrent mutations involve HMGA2 (high mobility group AT-hook 2) and fumarate hydratase (FH). Women with a first-degree relative who has fibroids face approximately 2.5 times the background risk, according to data from twin and family studies.

Hormonal Drivers

Estrogen and progesterone both promote fibroid growth by upregulating growth factors including insulin-like growth factor-1 (IGF-1) and epidermal growth factor (EGF). This explains two well-established clinical observations: fibroids rarely develop before menarche, and they typically shrink after menopause when ovarian estrogen production falls. Exogenous estrogen, such as estrogen-only hormone therapy after hysterectomy, may stimulate residual fibroid tissue or promote growth of undetected small fibroids.

A 2006 study in Obstetrics and Gynecology found that fibroids expressed higher concentrations of estrogen receptor-alpha and progesterone receptor than adjacent normal myometrium, providing a molecular explanation for their hormone sensitivity.

Environmental and Lifestyle Risk Factors

Several modifiable and non-modifiable exposures are associated with increased fibroid risk:

• Age: Risk increases steadily from the mid-twenties through the early fifties.
• Nulliparity: Women who have never been pregnant face higher risk. Each full-term pregnancy appears to reduce subsequent fibroid risk by approximately 20 to 25%, possibly because post-pregnancy uterine remodeling clears early fibroid foci.
• Obesity: Adipose tissue converts androgens to estrone, raising total estrogen exposure. A meta-analysis in Obesity Reviews found that each 10 kg/m² increase in BMI raised fibroid risk by roughly 21%.
• Diet: High red meat intake and low green vegetable consumption have been associated with increased fibroid risk in the Black Women's Health Study cohort (pubmed.ncbi.nlm.nih.gov/22028261/).
• Vitamin D deficiency: Low serum 25-hydroxyvitamin D is independently associated with fibroid prevalence. A 2013 study in Epidemiology reported that women with sufficient vitamin D levels had a 32% lower odds of fibroid diagnosis.
• Chemical exposures: Phthalates and other endocrine-disrupting chemicals have been studied in relation to fibroid risk, though causality is not yet established in human populations.

Symptoms of Uterine Fibroids

Approximately 50% of women with sonographically confirmed fibroids report no symptoms. When symptoms do occur, they depend heavily on fibroid number, size, and location.

Heavy Menstrual Bleeding

Heavy menstrual bleeding (menorrhagia) is the most reported symptom, affecting an estimated 30% of women with fibroids. Submucosal fibroids are particularly responsible. The mechanism involves distortion of the endometrial surface, impaired uterine contractility, and abnormal vascular patterns that prevent normal hemostatic responses during menstruation.

Clinically significant bleeding is defined as more than 80 mL per cycle. Passage of clots larger than a quarter, soaking through a pad or tampon in under an hour, or bleeding that persists beyond 7 days are all signals worth documenting at a clinical visit. Chronic heavy bleeding leads to iron-deficiency anemia in a substantial proportion of symptomatic women. One observational study in JAMA found that 40% of women seeking fibroid treatment had a hemoglobin below 10 g/dL.

Pelvic Pain and Pressure

Pelvic pressure, a sense of fullness, or dull lower abdominal discomfort are common with large intramural or subserosal fibroids. Acute pain occurs when a fibroid undergoes red degeneration (infarction), most commonly during pregnancy, or when a pedunculated fibroid twists on its stalk.

Dysmenorrhea (painful periods) is reported in roughly 25 to 30% of symptomatic fibroid patients. It may be difficult to distinguish fibroid-related dysmenorrhea from co-existing endometriosis, which affects up to 11% of reproductive-age women and frequently coexists with fibroids.

Bladder and Bowel Symptoms

Anterior subserosal fibroids pressing on the bladder cause urinary frequency, urgency, or incomplete bladder emptying. Posterior fibroids can compress the rectum, causing constipation or a sensation of rectal fullness. In rare cases, large fibroids compress a ureter, resulting in hydronephrosis detectable on imaging.

Reproductive Effects

Fibroids are found in 5 to 10% of women presenting with infertility. Submucosal fibroids distort the uterine cavity and are associated with reduced implantation rates and increased miscarriage risk. A Cochrane review (2017) concluded that removal of submucosal fibroids improves pregnancy rates, while the evidence for intramural fibroids that do not distort the cavity remains inconclusive.

Symptoms That Are Not Caused by Fibroids

Fibroids are sometimes blamed for symptoms that actually stem from other conditions. Irregular cycle timing (as distinct from heavy flow) is more often explained by anovulation, thyroid disease, or a coagulation disorder than by fibroids alone. Pelvic floor dysfunction, interstitial cystitis, and irritable bowel syndrome can all mimic fibroid-related bladder or bowel complaints. A thorough workup matters.

How Are Fibroids Diagnosed?

Pelvic Ultrasound

Transvaginal ultrasound (TVUS) is the first-line imaging modality. It identifies fibroid number, location, and size with good accuracy and no radiation. Sensitivity for detecting fibroids larger than 1 cm is approximately 95 to 98% with TVUS compared with surgical findings. The Society of Radiologists in Ultrasound recommends TVUS as the initial imaging choice for suspected uterine pathology.

MRI

Magnetic resonance imaging provides superior soft tissue contrast and is preferred for surgical planning, particularly before myomectomy or uterine artery embolization. MRI can distinguish fibroids from adenomyosis (a common mimic) with accuracy exceeding 85%, as reported in a 2012 meta-analysis in Radiology.

Saline Infusion Sonography and Hysteroscopy

When a submucosal component is suspected, saline infusion sonography (SIS) outlines the uterine cavity with fluid, improving the detection of intracavitary lesions. Hysteroscopy allows direct visualization and simultaneous resection of small submucosal fibroids during the same procedure.

Laboratory Tests

No blood test diagnoses fibroids. Labs are used to assess consequences: a complete blood count evaluates for anemia, thyroid-stimulating hormone rules out thyroid dysfunction as an alternative cause of menstrual changes, and coagulation studies (including von Willebrand factor antigen) are indicated for women with heavy bleeding since menarche.

Current Treatment Options: An Overview

Treatment is not mandatory for asymptomatic fibroids. For symptomatic disease, the choice among medical, procedural, and surgical options depends on symptom severity, fibroid characteristics, desire for future pregnancy, and patient preference.

Medical Management

GnRH agonists such as leuprolide acetate (Lupron) suppress ovarian estrogen production and reduce fibroid volume by 35 to 60% over 3 to 6 months. They are typically used short-term (3 to 6 months) because prolonged use causes bone mineral density loss. Adjunct "add-back" estrogen-progestin therapy mitigates bone effects.

GnRH antagonists offer a newer oral alternative. Elagolix (Orilissa, in combination with estradiol/norethindrone as Oriahnn) received FDA approval in 2020 for heavy menstrual bleeding due to uterine fibroids. Relugolix combination tablet (Myfembree) received FDA approval in May 2022 for the same indication and carries once-daily dosing with add-back therapy built into the formulation.

Levonorgestrel intrauterine system (LNG-IUS) reduces menstrual blood loss by roughly 74 to 97% in women with fibroids not distorting the cavity, based on data reviewed in a 2020 systematic review in BJOG.

Tranexamic acid (a non-hormonal antifibrinolytic) taken during menstruation reduces blood loss by approximately 40% and is FDA-approved for heavy menstrual bleeding.

Procedural Options

Uterine artery embolization (UAE) occludes the blood supply to fibroids, achieving symptom improvement in 85 to 90% of patients at one year in the EMMY trial (N=177). Focused ultrasound ablation (MR-HIFU) is a non-invasive option for women with a limited number of accessible fibroids.

Surgical Options

Myomectomy removes fibroids while preserving the uterus and is preferred for women desiring future fertility. Hysterectomy is definitive but ends childbearing. Minimally invasive routes (laparoscopic, robotic, hysteroscopic) are preferred over open surgery when feasible based on fibroid number and size. ACOG Practice Bulletin No. 228 states: "The route of hysterectomy should be chosen to minimize morbidity and to maximize patient outcomes and satisfaction, with vaginal hysterectomy preferred when feasible."

The decision framework below (to be illustrated by the HealthRX design team) maps symptom severity, fertility goals, and fibroid subtype to first-line versus second-line treatment pathways, drawing on the ACOG 2021 bulletin and the FDA approval timeline for GnRH antagonists through 2024.

When to See a Clinician

Certain presentations warrant prompt evaluation rather than a watch-and-wait approach. Contact a clinician if any of the following apply:

• Soaking more than one pad or tampon per hour for two or more consecutive hours
• Hemoglobin confirmed below 10 g/dL on a recent blood panel
• New pelvic mass detected by self-exam or imaging performed for another reason
• Rapid increase in uterine size over 6 to 12 weeks (rare, but may suggest sarcoma)
• Urinary retention or difficulty urinating

Routine surveillance with pelvic ultrasound every 6 to 12 months is appropriate for known asymptomatic fibroids, according to the AAFP's clinical guidance on abnormal uterine bleeding.

Living With Fibroids: What the Data Say About Long-Term Outcomes

Most fibroids remain stable or grow slowly. A longitudinal study tracking fibroid growth in 72 women over 12 months, published in Obstetrics and Gynecology, found that the median growth rate was 9% per year and that approximately one-third of fibroids actually shrank spontaneously during follow-up.

After menopause, fibroid regression is the rule rather than the exception. Women who reach natural menopause without requiring intervention most often experience resolution of bleeding symptoms within 12 to 24 months of their final menstrual period, as estrogen withdrawal removes the primary mitogenic stimulus.

Pregnancy outcomes with fibroids depend on fibroid size and location. Intramural fibroids larger than 5 cm are associated with a modestly increased risk of preterm birth and placental abruption, as documented in a 2017 meta-analysis in Human Reproduction Update. Women with known fibroids planning pregnancy should discuss imaging and timing with their gynecologist before conception.