Uterine fibroids grow primarily because of hormones, genetics, and a self-reinforcing biological loop where fibroid tissue produces its own growth signals. The typical fibroid grows about 9% in volume every six months, though individual fibroids vary wildly, with some shrinking and others more than doubling in size over the same period. Understanding what drives that growth can help you make sense of why fibroids behave the way they do and what factors you have some control over.
Estrogen and Progesterone Fuel Growth
Estrogen and progesterone are the primary drivers of fibroid growth. The mature cells inside fibroids express high levels of receptors for both hormones, which means they’re primed to respond every time your body produces them. This is why fibroids rarely develop before puberty and typically shrink after menopause, when hormone levels drop sharply.
What makes this more complicated is that fibroids don’t just respond to the estrogen your ovaries produce. Fibroid tissue contains an enzyme called aromatase that converts other hormones into estrogen locally, right inside the fibroid itself. Estrogen concentrations inside fibroid nodules are measurably higher than in the surrounding normal uterine muscle. This means a fibroid can partly sustain its own growth by manufacturing the hormone it needs, even when your overall estrogen levels are moderate.
Progesterone plays an equally important role. It stimulates cell division within fibroids and supports the production of structural proteins that give fibroids their dense, rubbery texture. Treatments that block progesterone’s effects can shrink fibroids significantly, which confirms how central this hormone is to their growth.
A Genetic Mutation Found in Most Fibroids
Between 50% and 80% of fibroids carry a mutation in a gene called MED12. This isn’t a mutation you inherit from your parents. It’s a somatic mutation, meaning it develops spontaneously in a single uterine muscle cell at some point during your life, and that cell then multiplies into a fibroid.
The MED12 mutation doesn’t necessarily make cells divide faster. Instead, it activates signaling pathways that increase collagen production, promote genomic instability, and push cells through their growth cycle more aggressively. It also blocks a cellular cleanup process called autophagy, which normally helps the body remove damaged or unnecessary cells. The result is a mass that accumulates dense, fibrous tissue over time rather than simply multiplying rapidly.
Growth Factors That Build the Fibroid’s Bulk
Much of a fibroid’s volume isn’t actually cells. It’s extracellular matrix: a dense scaffold of collagen, fibronectin, and other structural proteins that make fibroids feel firm and heavy. A key player here is a signaling molecule called TGF-beta3, which is expressed at high levels in fibroid tissue. TGF-beta3 does two things simultaneously: it stimulates the production of fibronectin (a major structural protein) and it directly promotes cell proliferation in both fibroid and normal uterine muscle cells.
Fibroid stem cells also respond strongly to insulin-like growth factors and other cytokines. These signals recruit new cells into the growing mass and help maintain the fibroid’s blood supply. The interplay between hormones, growth factors, and the structural matrix creates a feedback loop where the fibroid’s own environment encourages further growth.
Why Fibroids Grow During Pregnancy
Pregnancy creates a hormonal environment that can accelerate fibroid growth. The median fibroid volume increases by about 27% between the first and second trimesters. Smaller fibroids are more likely to grow than larger ones during pregnancy: 57% of fibroids below the median size grew, compared to 28% of larger fibroids.
This growth makes sense given that estrogen and progesterone levels climb dramatically during pregnancy. The increased blood flow to the uterus also delivers more nutrients and growth signals to existing fibroids. For most women, fibroids that grow during pregnancy shrink again in the months after delivery as hormone levels return to baseline.
Race and Fibroid Growth
Black women are three times more likely to be diagnosed with fibroids than white women, based on a study of nearly 1.9 million patients tracked over 14 years through Kaiser Permanente Northern California. Fibroids in Black women also tend to appear earlier and grow larger before diagnosis.
The biological reasons behind this disparity are not fully explained by a single factor. One contributor is vitamin D: African American women with serum vitamin D levels at or above 20 ng/mL had significantly slower fibroid growth over 18 months compared to those below that threshold. Since darker skin produces less vitamin D from sunlight, and vitamin D deficiency is more common among Black Americans, this nutritional gap may partly explain the difference in fibroid burden. Progesterone receptor levels in fibroid tissue also vary by ethnicity, suggesting that genetic differences in hormone sensitivity play a role.
Environmental Chemicals That May Promote Growth
Certain industrial chemicals that mimic or interfere with your hormones appear to increase fibroid risk. A meta-analysis of 14 studies involving nearly 5,800 women found that higher exposure to DEHP, a phthalate commonly found in plastics, food packaging, and personal care products, was associated with a 61% increased risk of fibroids. Both low and high molecular weight phthalate mixtures showed smaller but statistically significant associations with fibroid occurrence as well.
Phthalates act as endocrine disruptors, meaning they can interfere with how your body produces, processes, or responds to hormones like estrogen. Given that estrogen is the primary fuel for fibroid growth, chronic low-level exposure to these chemicals may create a hormonal environment that favors fibroid development over time. You encounter phthalates through plastic food containers, vinyl flooring, fragranced products, and some cosmetics.
Vitamin D and Fibroid Growth Rate
Low vitamin D levels are consistently linked to faster fibroid growth. The clearest data comes from studies of African American women, where those with vitamin D levels at or above 20 ng/mL (a threshold many doctors consider the minimum for bone health) had meaningfully slower fibroid growth over 18 months compared to women below that level.
Vitamin D appears to work against fibroids through several mechanisms: it can suppress cell proliferation, reduce the production of fibrous structural proteins, and inhibit some of the signaling pathways that fibroids rely on. While research hasn’t yet established a specific dose that prevents fibroid growth, maintaining adequate vitamin D levels is a low-risk strategy. A simple blood test can tell you where your levels stand, and supplementation is inexpensive if you’re deficient.
How Fast Fibroids Actually Grow
The median growth rate for untreated fibroids in premenopausal women is about 9% in volume every six months, with Black and white women showing similar rates (12% vs. 10% per six months, respectively). But that median hides enormous variation. Individual fibroids ranged from shrinking by 89% to growing by 138% over the same period.
This unpredictability is one of the most frustrating aspects of fibroids. Some women have fibroids that stay small for years, while others experience rapid growth that leads to heavy bleeding, pelvic pressure, or fertility problems within months. The rate depends on the fibroid’s specific genetic makeup, its hormone receptor density, its blood supply, and the overall hormonal and nutritional environment in your body. Two fibroids in the same uterus can behave completely differently.