Uterine fibroids develop when a single muscle cell in the uterine wall picks up a genetic mutation and begins growing into a firm, rounded mass under the influence of hormones. There isn’t one single cause. Fibroids result from a chain of events: a genetic trigger, hormonal fuel, and a collection of risk factors that make the whole process more likely.
Each Fibroid Starts From One Cell
Every fibroid is a clone. It traces back to a single stem cell in the muscular wall of the uterus (the myometrium) that acquired a mutation and started dividing abnormally. This is why a woman can have multiple fibroids of different sizes, each behaving independently. Each one grew from its own mutated cell, with its own genetic fingerprint.
The most common mutation found in fibroids involves a gene called MED12, which helps regulate how cells read their DNA. Roughly 70% of fibroids carry a mutation in this gene. Research using whole-genome sequencing found that no other gene mutations were present alongside MED12 in these tumors, suggesting that this single mutation may be enough to start a fibroid on its own. The mutation has been found only in fibroid tissue, not in the surrounding healthy uterine muscle, confirming it’s an acquired change rather than something inherited across the whole body.
Hormones Are the Fuel, Not the Spark
A mutation alone doesn’t grow a fibroid. Hormones do the heavy lifting. Progesterone is the primary driver of fibroid growth, and estrogen plays a critical supporting role by making fibroid cells more responsive to progesterone. Here’s how that works: estrogen stimulates fibroid cells to produce more progesterone receptors. Progesterone then binds to those receptors and triggers the cells to multiply and survive longer than they normally would. Without estrogen priming those receptors, progesterone can’t do its job, and the tumor doesn’t grow.
This hormonal dependency explains several patterns that women notice. Fibroids tend to grow during the reproductive years when estrogen and progesterone cycle monthly. They often enlarge during pregnancy, when both hormones surge. And they typically shrink after menopause, when hormone levels drop. It also explains why fibroids are rare before puberty.
The growth mechanism involves a kind of teamwork between cells. Mature fibroid cells, which are rich in progesterone receptors, respond to progesterone by releasing chemical signals including growth factors. These signals then reach nearby stem-like fibroid cells that lack their own hormone receptors, pushing them to proliferate. So the tumor essentially feeds its own growth through internal communication.
Why Fibroids Feel Hard and Dense
The “fibroid” name comes from the enormous amount of fibrous tissue these tumors produce. Both the mutated muscle cells and the surrounding supportive cells lay down excessive quantities of extracellular matrix, a scaffolding of collagen and other structural proteins. A growth factor called TGF-beta3 is a key player here. It ramps up the production of matrix proteins while simultaneously reducing the enzymes that would normally break those proteins down. The result is a dense, rubbery mass that can grow much larger than the small cluster of dividing cells at its core.
This fibrous buildup is what makes fibroids feel firm during a pelvic exam and what gives them their characteristic appearance on ultrasound. It’s also part of why they cause symptoms like heavy bleeding and pelvic pressure: the stiff tissue distorts the uterine wall and cavity in ways that soft tissue would not.
Vitamin D Deficiency and Fibroid Growth
Low vitamin D levels are consistently linked to a higher risk of developing fibroids, and laboratory research offers a plausible explanation. Vitamin D reduces fibroid cell proliferation, triggers programmed cell death in fibroid cells, and directly counteracts the fibrosis process driven by TGF-beta3. In cell studies, vitamin D suppressed the production of collagen, fibronectin, and other matrix proteins that give fibroids their bulk. It also reduced the expression of both estrogen and progesterone receptors in fibroid cells, potentially cutting off the hormonal fuel supply.
In animal models, vitamin D treatment shrank existing fibroids. These findings are significant because vitamin D deficiency is extremely common, particularly among Black women, who also develop fibroids at higher rates and at younger ages than white women. While vitamin D status alone doesn’t explain racial disparities in fibroid prevalence, it appears to be one meaningful piece of the puzzle.
How Body Fat Influences Fibroid Risk
Obesity increases fibroid risk through multiple pathways. Fat tissue is hormonally active. It contains an enzyme called aromatase that converts other hormones into estrogen, and women with more visceral fat (the deep abdominal kind) show higher aromatase activity. This means more circulating estrogen, which fuels the hormonal cycle that drives fibroid growth.
But the connection goes deeper than just extra estrogen. Fat cells communicate directly with the stem cells in the uterine wall. Research shows that signals from fat cells create a pro-inflammatory environment with high levels of reactive oxygen species, molecules that damage DNA. This oxidative stress can cause the very mutations that transform a normal uterine stem cell into a fibroid-initiating cell. So excess body fat may contribute both to the initial spark (DNA damage) and the ongoing fuel (elevated estrogen).
Environmental Chemicals and Early Life Exposure
Certain industrial chemicals that mimic or interfere with hormones, known as endocrine disruptors, are associated with increased fibroid risk. The most studied include phthalates (found in plastics, personal care products, and food packaging), BPA (found in some plastics and can linings), and DES, a synthetic estrogen prescribed to pregnant women from the 1940s through the 1970s. BPA promotes fibroid cell proliferation by activating growth-signaling pathways. Phthalates appear to alter the regulation of small RNA molecules that control gene activity in fibroid tissue.
Timing of exposure matters enormously. The developing uterus is especially vulnerable. Animal studies show that exposure to endocrine disruptors during fetal development or shortly after birth increases the number, size, and frequency of fibroids in adulthood. This happens through epigenetic reprogramming: the chemicals don’t change the DNA sequence itself, but they alter the chemical tags that control which genes are turned on or off. These changes persist for life, essentially resetting the uterine tissue’s vulnerability to fibroid formation long before a woman reaches reproductive age.
Epigenetic Changes in Fibroid Tissue
Beyond outright mutations, fibroids carry a distinct pattern of epigenetic modifications compared to normal uterine muscle. Tumor suppressor genes, which normally keep cell growth in check, are silenced in fibroid tissue through a process called hypermethylation, where chemical tags are added to the gene’s control region to shut it down. The stem cells that give rise to fibroids also show suppressed activity in genes related to normal muscle function and hormone response, which helps them maintain their stem-like, rapidly dividing state instead of maturing into ordinary muscle cells.
This means fibroids aren’t just about having the wrong genes. They’re also about having normal genes switched off at the wrong time. Environmental exposures, hormonal fluctuations, and inflammatory signals can all influence these epigenetic patterns, which helps explain why fibroid risk is shaped by so many different factors working together rather than any single cause.
Putting the Causes Together
Fibroid development follows a multi-step process. First, a stem cell in the uterine wall sustains a DNA mutation, most commonly in the MED12 gene. This can happen during normal monthly hormone cycles, when stem cells divide to prepare the uterus for potential pregnancy, or it can be made more likely by oxidative stress, obesity, or early-life chemical exposures. Second, the mutated cell begins growing under the influence of progesterone (enabled by estrogen), gradually building a mass of dense, fibrous tissue. Third, factors like vitamin D deficiency, excess body fat, and ongoing hormone exposure determine how fast and how large the fibroid grows.
No single factor is sufficient on its own. A woman with a MED12 mutation but very low hormone levels (after menopause, for example) won’t grow a significant fibroid. A woman with high estrogen but no initiating mutation won’t develop one either. It’s the combination of genetic susceptibility, hormonal environment, and modifiable risk factors that determines whether fibroids develop and how much trouble they cause.