Uterine fibroids grow primarily because of estrogen and progesterone, the two main reproductive hormones. But hormones are only part of the story. Genetics, body weight, environmental chemicals, and even vitamin D levels all influence whether fibroids stay small or keep expanding. On average, fibroids increase in volume by 20 to 30 percent per year before menopause, though the rate varies widely from person to person.
Estrogen and Progesterone Fuel Growth
Fibroid tissue contains far more hormone receptors than normal uterine muscle. The expression of estrogen receptors in fibroids is 1.8 to 2.6 times higher than in the surrounding tissue. This means fibroids are essentially primed to absorb and respond to estrogen and progesterone more aggressively than the rest of the uterus.
Estrogen drives fibroid growth in two ways. It switches on genes that control cell division and triggers rapid signaling cascades that push cells to multiply. It also stimulates the production of growth factors that cause the fibroid to build up its internal scaffolding, a dense mesh of collagen and other structural proteins called the extracellular matrix. This matrix is what gives fibroids their firm, rubbery texture and accounts for much of their bulk.
Progesterone works alongside estrogen rather than opposing it. High levels of progesterone ramp up markers of cell proliferation and amplify the effects of growth factors that estrogen already set in motion. Progesterone also blocks the natural cell-death process that would normally keep tissue growth in check. Together, these two hormones create a cycle where fibroid cells multiply faster than they should and survive longer than they should.
A Single Gene Mutation Starts Most Fibroids
About 70 percent of fibroids trace back to a mutation in a gene called MED12, which helps regulate how cells read their DNA instructions. When this gene is altered in a single uterine muscle cell, it sets off a chain reaction. The mutated cell begins converting an amino acid called tryptophan into a compound that activates a receptor involved in cell growth. This drives the cell to proliferate, resist normal death signals, and eventually form a fibroid nodule that then becomes dependent on progesterone to keep growing.
This is why fibroids are considered monoclonal, meaning each one starts from a single rogue cell. You can have multiple fibroids in the same uterus, and each one likely arose from its own independent mutation. The genetic trigger explains why fibroids are so common (affecting up to 70 to 80 percent of women by age 50) and why they tend to run in families.
Body Weight and Insulin Resistance
Fat tissue is not just storage. It actively produces estrogen through an enzyme called aromatase. The more adipose tissue you carry, the more estrogen circulates in your blood. Obesity also raises insulin levels, and insulin has its own effect on estrogen: it increases androgen production in the ovaries (which gets converted to estrogen) and decreases the liver’s production of a protein that binds estrogen and keeps it inactive. The result is higher levels of free, active estrogen available to feed fibroids.
This relationship helps explain why maintaining a healthy weight is one of the few modifiable factors linked to fibroid growth. It also explains why fibroids shrink less after menopause in women with a higher BMI. A 10-year study found that women with a BMI under 25 experienced significantly greater fibroid shrinkage after menopause (25 percent reduction) compared to women with a BMI of 25 or above (15.7 percent reduction).
Environmental Chemicals That Mimic Hormones
Endocrine-disrupting chemicals, synthetic substances that interfere with your hormone system, are increasingly linked to fibroid growth. The most studied is DEHP, a phthalate found in flexible plastics, food packaging, and personal care products. Every study examining DEHP metabolites has found a positive association with fibroid outcomes, including increased fibroid risk, severity, and uterine volume.
Bisphenol A (BPA), found in some plastics and can linings, has also been shown to increase the proliferation of human fibroid cells in laboratory studies. Other chemicals associated with fibroid risk include parabens (common preservatives in cosmetics), organophosphate esters (used as flame retardants), and certain alternative plasticizers marketed as “BPA-free” replacements. These chemicals can mimic estrogen in the body, effectively adding to the hormonal signals that drive fibroid growth.
How Fast Fibroids Actually Grow
Growth rates vary enormously. The median volume increase is about 35 percent per year in premenopausal women, but individual fibroids in the same study ranged from shrinking by 7 percent to growing by 210 percent in a single year. Age matters too: women under 35 saw an average volume increase of 70 percent per year, compared to 30 percent in women over 35.
This wide range means that some fibroids sit quietly for years while others double in size within months. There is no reliable way to predict which fibroids will grow rapidly, which is why monitoring with periodic ultrasound is standard for fibroids that aren’t causing symptoms.
Fibroids During Pregnancy
Pregnancy floods the body with estrogen and progesterone, so you might expect fibroids to explode in size. In reality, only 22 to 32 percent of fibroids grow during pregnancy. Among those that do, growth is concentrated almost entirely in the first trimester, particularly the first 10 weeks. The mean volume increase is modest: about 12 percent, with a maximum of 25 percent. Most fibroids stabilize or even shrink slightly in the second and third trimesters.
What Happens After Menopause
When estrogen and progesterone levels drop at menopause, most fibroids gradually shrink. The fastest shrinkage occurs in the first two years, then slows considerably. However, not all fibroids respond equally. Women with four or more fibroid nodules saw significantly less shrinkage over 10 years (15.2 percent) compared to women with a single fibroid (26.3 percent). For women with multiple large fibroids, the shrinkage rate didn’t meaningfully accelerate in those early postmenopausal years the way it did for women with fewer, smaller fibroids.
Diet, Alcohol, and Vitamin D
Current alcohol drinkers have about a 33 percent higher risk of fibroids compared to women who have never consumed alcohol. The association is strongest with long-term use (20 years or more) and higher intake, particularly beer at seven or more drinks per week. Alcohol may contribute by raising circulating estrogen levels, which is a well-documented effect of regular drinking.
Vitamin D appears to work in the opposite direction. Women with sufficient vitamin D levels (above 20 ng/mL) had an estimated 32 percent lower odds of having fibroids compared to women with insufficient levels. This protective association held for both small and large fibroids, suggesting vitamin D may inhibit both the initial formation of fibroids and their continued growth. In animal models, the active form of vitamin D reduced fibroid volume and inhibited the production of the extracellular matrix that makes fibroids dense and bulky. Vitamin D deficiency is notably more common in Black women, who also develop fibroids more frequently and at younger ages.
Why Some Fibroids Grow Faster Than Others
No single factor determines a fibroid’s growth trajectory. A fibroid in a younger woman with higher circulating hormones, a MED12 mutation driving its internal growth pathway, elevated BMI increasing free estrogen, and low vitamin D levels faces a very different biological environment than a small fibroid in a lean, older woman approaching menopause. The combination of genetic susceptibility, hormonal exposure, metabolic health, and environmental inputs creates a unique growth profile for every fibroid. This is why two women of the same age can have dramatically different experiences, with one fibroid staying marble-sized for a decade while another grows to the size of a grapefruit in a year.