Fibroids grow primarily because they are unusually sensitive to the hormones estrogen and progesterone. Fibroid tissue contains significantly more hormone receptors than normal uterine muscle, and those receptors also have a higher affinity for estrogen. This means fibroids don’t just respond to your body’s hormones; they respond more intensely than the tissue around them. But hormones are only part of the story. Genetics, growth factors, diet, body composition, and environmental chemicals all play a role in how fast fibroids expand and whether new ones develop.
How Hormones Fuel Fibroid Growth
Estrogen and progesterone are the two main drivers. Both types of estrogen receptors are found in greater abundance in fibroid cells compared to the surrounding uterine muscle. The receptors also bind estrogen more tightly, which amplifies the hormonal signal inside the cell. When estrogen reaches a fibroid cell at high enough concentrations, it rapidly triggers a chain of internal signals that push the cell to divide. In healthy uterine muscle, the same signaling pathways actually inhibit cell growth, but in fibroids, those protective brakes are altered.
Progesterone plays an equally important role. Roughly 70% of fibroids carry a specific genetic mutation (more on that below) that makes them grow in a progesterone-dependent manner. This is why fibroids tend to enlarge during the reproductive years, when both hormones cycle at their highest levels, and why they typically shrink after menopause, when estrogen and progesterone drop sharply. Women on hormone replacement therapy after menopause sometimes find that their fibroids don’t shrink as expected, because the hormonal support continues.
The Genetic Trigger Behind Most Fibroids
About 70% of fibroids share a mutation in a gene called MED12, which helps regulate how cells read their DNA. This single mutation appears to be the initiating event: it turns a normal uterine muscle cell into one that starts growing out of control. Research published in JCI Insight found that the MED12 mutation activates a specific metabolic pathway involving the amino acid tryptophan. Fibroid cells with this mutation ramp up an enzyme that converts tryptophan into a compound called kynurenine, which then switches on a receptor that promotes further cell growth.
The mutation also drives changes in the structural scaffolding around fibroid cells, known as the extracellular matrix. Fibroids aren’t just clumps of multiplying cells. Much of their bulk comes from this dense, fibrous matrix, and the MED12 mutation activates pathways that increase its production. This is why fibroids feel firm and rubbery, and why they can grow to substantial sizes even when cell division itself isn’t particularly rapid.
Growth Factors That Build Bulk
Beyond hormones and genetics, several growth-signaling proteins contribute to fibroid expansion. One of the most important is transforming growth factor beta (TGF-beta), which stimulates cells to produce more extracellular matrix proteins. Other growth factors, including insulin-like growth factor and platelet-derived growth factor, do the same. These proteins don’t just make cells divide; they tell cells to manufacture the dense connective tissue that makes up much of a fibroid’s volume.
The extracellular matrix itself acts as a storage depot for these growth factors. As the matrix expands, it traps more signaling molecules, which can be rapidly released to stimulate further growth. This creates a self-reinforcing loop: more matrix means more stored growth signals, which means more matrix production, which means a steadily enlarging fibroid. At low concentrations, TGF-beta can also remove its own braking effect on cell division while stimulating cells to produce and respond to additional growth signals, further accelerating the cycle.
How Fast Fibroids Actually Grow
Fibroids don’t grow at a steady, predictable rate. A study that tracked 101 fibroids over 12 months using MRI scans found that 37 of them experienced “growth spurts,” defined as expanding 30% or more in volume over a three-month period and then slowing down. This means growth is often episodic rather than linear. You might have a fibroid that barely changes for months and then suddenly gets noticeably larger.
Larger fibroids (over 5 cm in diameter) tended to have less dramatic short-term fluctuation than smaller ones. This doesn’t mean large fibroids stop growing, but their growth appears more gradual relative to their existing size. For smaller fibroids, the unpredictability of growth spurts can make it harder to forecast how they’ll behave over time.
What Happens During Pregnancy
Pregnancy creates a hormone-rich environment that can accelerate fibroid growth, but the pattern is more complex than a simple nine-month increase. Fibroids most commonly enlarge during the first trimester, when immune cells in the uterus called natural killer cells are most active. These cells are associated with fibroid growth and are at their peak early in pregnancy.
During the second trimester, 51% to 56% of women with fibroids see an increase in total fibroid volume. By the third trimester, the trend reverses: 50% to 54% of women experience a decrease. Because sex hormones, immune signals, and blood pressure hormones all rise throughout pregnancy, researchers believe it’s the shifting balance of these factors, not any single one, that determines whether fibroids grow or shrink at each stage.
Vitamin D Deficiency and Fibroid Growth
Low vitamin D levels are strikingly common among women with fibroids. In one prospective ultrasound study, 73% of participants had deficient vitamin D (below 20 ng/mL). Women whose levels were at or above 20 ng/mL had an estimated 9.7% reduction in fibroid growth rate compared to those below that threshold. At levels of 30 ng/mL or above, there was a 22% reduction in the risk of developing new fibroids and a 32% increase in the likelihood that existing fibroids would shrink or disappear.
These findings don’t prove that taking vitamin D supplements will shrink fibroids, but they establish a consistent link between adequate vitamin D and slower fibroid growth. Since deficiency is so prevalent in this population, it’s a factor worth paying attention to.
Diet, Body Weight, and Alcohol
What you eat appears to influence fibroid risk, though the effects are gradual rather than dramatic. A large review found that women who ate four servings of fruits and vegetables per day had a lower risk of developing fibroids than those who ate one serving. The likely mechanisms are twofold: dietary fiber helps the body clear excess estrogen through the digestive tract, and a produce-heavy diet tends to support a lower body mass index. Higher BMI raises circulating estrogen levels, which gives fibroids more hormonal fuel.
Red meat and processed meat (including ham) are associated with higher fibroid risk. In multiple studies, women with fibroids reported eating more of these foods than women without fibroids. Alcohol also appears to play a role, particularly at higher intake levels, because it alters hormone metabolism in ways that can promote fibroid formation. Calcium-rich, low-fat dairy products have been linked to a modestly lower risk, possibly because of their vitamin D content.
Environmental Chemicals That Mimic Estrogen
Certain industrial chemicals can act like weak estrogen in the body, and growing evidence links them to fibroid risk. Phthalates, found in plastics, personal care products, and food packaging, are among the most studied. A pooled analysis of observational studies found that women with higher exposure to the common phthalate DEHP had a 61% increased risk of uterine fibroids compared to women with lower exposure. Both low-molecular-weight and high-molecular-weight phthalate mixtures were associated with a small but statistically significant increase in risk (about 8% each).
These chemicals are difficult to avoid entirely because they’re so widespread in consumer products. Reducing exposure by choosing glass or stainless steel food containers, avoiding heavily fragranced products, and minimizing plastic food packaging are practical steps, though the degree to which these changes affect existing fibroids isn’t well established.
Why Fibroids Shrink After Menopause
Once estrogen and progesterone levels drop during menopause, fibroids lose the hormonal stimulation they depend on. For many women, this leads to a gradual reduction in fibroid size over the months and years following menopause. The shrinkage isn’t universal, though. Some fibroids remain the same size or decrease only slightly, particularly in women using hormone replacement therapy, which partially restores the hormonal environment that supports fibroid growth. Fibroids rarely disappear completely, but they often become small enough that they no longer cause symptoms.