Fibroids develop when a single smooth muscle cell in the uterine wall picks up a genetic mutation and begins multiplying into a dense, rubbery growth of cells and connective tissue. Hormones, especially estrogen and progesterone, fuel that growth, which is why fibroids almost always appear during the reproductive years and tend to shrink after menopause. But the full picture involves far more than hormones alone. Body weight, blood pressure, vitamin D levels, chemical exposures, and even the timing of your first period all shape your risk in ways researchers are still piecing together.
How a Fibroid Starts at the Cellular Level
The uterine wall is made of smooth muscle. The prevailing model of fibroid development holds that a single muscle stem cell undergoes a genetic change that turns it into a tumor-initiating cell. That one cell then seeds a clonal growth, meaning every cell in a fibroid traces back to the same ancestor.1SpringerOpen. Human Myometrial and Uterine Fibroid Stem Cell-Derived Organoids for Intervening the Pathophysiology of Uterine Fibroid As the growth expands, the cells produce large amounts of extracellular matrix, a kind of biological scaffolding made mainly of collagen. That matrix is a big part of what makes fibroids feel firm and what gives them their bulk.
Despite being the most common pelvic tumor in women, with roughly 70% of women developing at least one by age 50, fibroids remain surprisingly difficult to study in the lab. The mutant cells that drive a fibroid tend to get crowded out by normal cells once tissue is placed in a culture dish, which has slowed down research into exactly how fibroids form and what might prevent them.2PubMed Central. Engineered MED12 mutations drive uterine fibroid-like transcriptional and metabolic programs by altering the 3D genome compartmentalization
The MED12 Mutation and Other Genetic Drivers
About 70% of fibroids carry a mutation in a gene called MED12.2PubMed Central. Engineered MED12 mutations drive uterine fibroid-like transcriptional and metabolic programs by altering the 3D genome compartmentalization This gene helps regulate how other genes are turned on and off. When MED12 is mutated, it activates signaling pathways that promote cell growth and remodel the tissue architecture, pushing the fibroid to expand and stiffen.3Journal of Endometriosis and Uterine Disorders. MED12 somatic mutation promotes human uterine leiomyoma’s growth by trigging canonical Wnt4/β-catenin and TGF-β signaling through the EMT pathway These are somatic mutations, meaning they arise in the uterine tissue itself rather than being inherited from a parent. You do not pass a fibroid-causing MED12 mutation to your children.
That said, family history does matter. If your mother or sister had fibroids, your risk is higher. The inherited component is not one specific “fibroid gene” but rather a combination of genetic variants that influence things like how your body processes hormones, how your immune system handles inflammation, and how readily your uterine tissue remodels itself. Epigenetic changes, alterations in how genes are read without changing the DNA sequence itself, also play a role. Researchers have found that shifts in DNA methylation patterns in fibroid tissue can switch certain genes on or off in ways that promote tumor formation.4PubMed Central. Epigenetic Regulation in Uterine Fibroids-The Role of Ten-Eleven Translocation Enzymes and Their Potential Therapeutic Application
Why Estrogen and Progesterone Matter So Much
Both estrogen and progesterone are critical to fibroid growth, and recent research has shifted the spotlight onto progesterone more than earlier work anticipated. For decades, estrogen was treated as the primary hormonal driver. That made intuitive sense: fibroids grow during the reproductive years when estrogen is high and shrink after menopause when it drops. But studies now show that progesterone is not merely a bystander. It interacts with receptors on fibroid cells and activates signaling pathways that boost tumor growth and cell survival.5PubMed Central. Progesterone signaling in uterine fibroids: Molecular mechanisms and therapeutic opportunities
When both hormones are present together, the effect is more than additive. High-dose progesterone ramps up markers of cell proliferation, and combined with estrogen, the two hormones enhance growth-factor signals that amplify each other’s effects.6PubMed Central. The estrogen-progestogen-oxidative stress network in uterine fibroids: mechanistic insights and therapeutic opportunities This is part of the reason fibroids can grow rapidly during pregnancy, when both hormone levels surge, and why some drug treatments for fibroids work by suppressing progesterone signaling.
The hormones do not act on fibroid cells in isolation. Mature cells in the surrounding uterine muscle respond to estrogen and progesterone by releasing growth signals that reach nearby stem cells, essentially recruiting them into fibroid growth. This paracrine signaling, where one cell type sends chemical messages to a neighboring cell type, helps explain how a tiny cluster of mutant cells can balloon into a growth that weighs several pounds.7PubMed Central. Paracrine activation of WNT/β-catenin pathway in uterine leiomyoma stem cells promotes tumor growth
The Extracellular Matrix and Why Fibroids Feel Hard
A fibroid is not just an overgrowth of muscle cells. A large fraction of its mass comes from the extracellular matrix, particularly collagen and other structural proteins deposited between the cells. A signaling molecule called TGF-β is overproduced in fibroid tissue, and it drives excessive matrix production and storage.8PubMed Central. Role of Transforming Growth Factor β in Uterine Fibroid Biology The resulting stiffness is not just cosmetic. It creates a mechanical environment that further promotes cell growth, setting up a feedback loop: stiffer tissue encourages more matrix production, which stiffens the tissue further. This is why some fibroids grow quickly while others remain stable for years. The balance between cell proliferation and matrix buildup varies from one fibroid to another, even in the same uterus.
Race, Ethnicity, and Fibroid Risk
Fibroids affect women of all racial and ethnic backgrounds, but the disparity experienced by Black women is striking. Ultrasound evidence shows that more than 80% of Black women and about 70% of white women will develop fibroids by age 50. Black women also face roughly three times the age-adjusted risk of developing fibroids compared to white women, are diagnosed at younger ages, are more likely to have symptoms, and tend to have different responses to treatment.9PubMed Central. The Health Disparities of Uterine Fibroids for African American Women: A Public Health Issue
Researchers do not believe a single explanation accounts for the gap. Some of the disparity likely reflects genetic differences in how uterine tissue responds to hormones and growth signals. But socioeconomic factors also contribute. Black women in the United States have disproportionately higher rates of vitamin D deficiency (discussed below), higher average body mass index, greater exposure to certain environmental chemicals, and more barriers to early diagnosis and treatment. Disentangling the biological from the social is an active area of research, and the honest answer is that no one has fully sorted it out yet.
Body Weight and Insulin Resistance
Higher body weight is one of the most consistently identified modifiable risk factors for fibroids. A systematic review of modifiable factors found that a high body mass index was the strongest risk factor for fibroid development.10PubMed Central. The association of insulin resistance and obesity with uterine fibroids in non-diabetic populations: a cross-sectional study Fat tissue produces estrogen, so carrying extra weight means higher circulating estrogen levels, which feeds hormone-dependent growth. But weight affects fibroids through another pathway, too: insulin resistance. Even in women without diabetes, higher insulin resistance is independently associated with fibroids.10PubMed Central. The association of insulin resistance and obesity with uterine fibroids in non-diabetic populations: a cross-sectional study
A large genetic study using a method called Mendelian randomization, which uses inherited gene variants as a natural experiment to test cause-and-effect, identified insulin resistance and the hormone leptin as likely mediators of the link between obesity and fibroids.11PLOS Medicine. Obesity and risk of female reproductive conditions: A Mendelian randomisation study Leptin, which is produced in proportion to body fat, has been shown to promote cell growth in uterine tissue. Together, these findings suggest the connection between weight and fibroids is not just about estrogen from fat cells but involves a broader metabolic environment.
Early Periods and Reproductive History
Getting your first period at a younger age is linked to a higher chance of developing fibroids. A study that used standardized ultrasound screening found that for each year later a woman started her period, her risk of fibroids dropped by about 13%. The association was even stronger for women who went on to develop multiple fibroids.12PubMed Central. Association of age at menarche with increasing number of fibroids in a cohort of women who underwent standardized ultrasound assessment The explanation is straightforward: earlier periods mean more cumulative years of exposure to cycling estrogen and progesterone.
Pregnancy, on the other hand, appears to be protective. Most studies find that women who have given birth have a 20 to 50% lower risk of fibroids compared to those who have not, and the risk tends to drop further with each additional birth.13PubMed Central. Epidemiology of Uterine Fibroids – From Menarche to Menopause The mechanism is not entirely clear, but one hypothesis is that the massive remodeling the uterus undergoes during pregnancy and postpartum recovery clears away small, early-stage fibroids. Another is that breastfeeding suppresses ovulation, reducing hormone exposure. Neither explanation has been definitively confirmed.
Vitamin D Deficiency
Low vitamin D levels have emerged as a consistent risk factor. In one study, women with sufficient vitamin D had an estimated 32% lower odds of fibroids compared with women who were vitamin D insufficient.14PubMed Central. Vitamin d and the risk of uterine fibroids Laboratory work has shown that vitamin D can inhibit fibroid cell growth, reduce the production of extracellular matrix, and trigger programmed cell death in fibroid tissue. These findings have generated excitement because vitamin D is cheap and widely available, but clinical trials testing whether supplementation actually prevents or shrinks fibroids are still limited. The observational link is strong enough to be worth paying attention to, though it has not yet been proven that taking supplements will change your outcome.
Vitamin D deficiency is also far more common in Black women due to differences in skin melanin content, which reduces vitamin D synthesis from sunlight. Some researchers have proposed that vitamin D deficiency could account for part of the racial disparity in fibroid prevalence, though it is unlikely to be the whole story.
Diet, Alcohol, and Physical Activity
A systematic review of modifiable risk factors found that a diet high in fruits and vegetables was one of the strongest protective factors against fibroid development, alongside vitamin D intake. The likely reason is that plant-heavy diets are rich in antioxidants, anti-inflammatory compounds, and fiber, which helps regulate estrogen by promoting its excretion through the digestive system. Alcohol, by contrast, has been linked to increased risk in some studies, potentially because it raises estrogen levels and impairs the liver’s ability to clear excess hormones.
Physical activity also appears to help. A study adjusting for body mass index and other risk factors found that women in the highest category of physical activity, equivalent to roughly seven or more hours per week, were about 40% less likely to have fibroids compared with the least active women. The benefit followed a dose-response pattern: more activity correlated with lower risk, and the trend held for both Black and white women. The analysis suggested exercise was more strongly associated with preventing new fibroids from forming than with slowing the growth of existing ones.15Oxford Academic (American Journal of Epidemiology). Association of Physical Activity with Development of Uterine Leiomyoma
Endocrine-Disrupting Chemicals
Chemicals in the environment that mimic or interfere with hormone signaling have been linked to fibroid risk. Epidemiological studies point to phthalates (found in plastics, cosmetics, and fragrances) and bisphenol A (found in some food packaging and receipt paper) as particularly concerning. One common phthalate, DEHP, has been associated with increased fibroid risk and severity, and laboratory studies show that both DEHP and bisphenol A can affect biological pathways critical to fibroid formation.16PubMed Central. The role of endocrine-disrupting chemicals in uterine fibroid pathogenesis Per- and polyfluoroalkyl substances (PFAS), sometimes called “forever chemicals,” and parabens used as preservatives in personal care products are also under investigation for their role in uterine disorders, including fibroids.17PubMed. The impact of endocrine-disrupting chemicals on uterine diseases
This area is genuinely hard to study because everyone is exposed to some level of these chemicals and the effects may accumulate over decades. Proving that a specific chemical caused a specific fibroid in a specific person is essentially impossible with current methods. But the pattern across cell studies, animal work, and human population data is consistent enough that many reproductive health researchers consider endocrine disruptors a plausible contributor to fibroid risk, particularly for communities with higher exposure levels.
High Blood Pressure and Vascular Health
Fibroids and hypertension show up together more often than chance would predict, and the relationship may be more than a coincidence. Both conditions involve abnormal behavior of smooth muscle cells and dysfunction of the blood vessel lining. Chronically high blood pressure can damage the smooth muscle cells and blood vessels within the uterine wall, potentially creating conditions favorable to the kind of abnormal cell growth that seeds a fibroid.18Scientific Reports. Association between uterine fibroids and hypertension among women of reproductive age in Eastern Sudan: a matched case–control study Shared mechanisms including the renin-angiotensin system, estrogen signaling, and endothelial dysfunction all likely play roles linking fibroids and vascular conditions like hypertension and atherosclerosis.19Reproduction. Vascular biology of uterine fibroids: connecting fibroids and vascular disorders
Whether treating high blood pressure reduces fibroid risk is unknown. But the overlap underscores that fibroids are not just a reproductive issue. They sit at the intersection of hormonal, metabolic, and cardiovascular health.
Stress and Inflammation
The idea that stress contributes to fibroids is biologically plausible, though hard to prove in the way a controlled trial can prove a drug works. Chronic stress activates the body’s stress-response system, which releases cortisol and in turn disrupts the normal cycling of estrogen and progesterone. Since fibroids are hormone-dependent, those fluctuations could encourage growth.20PubMed Central. The Association between Self-Reported Major Life Events and the Presence of Uterine Fibroids Stress also promotes systemic inflammation, which may create a tissue environment more hospitable to abnormal growth. This pathway is difficult to isolate because stress often co-occurs with other risk factors like poor sleep, lower physical activity, and less access to healthcare.
Oral Contraceptives and Fibroid Risk
Given that fibroids are hormone-driven, you might wonder whether taking hormonal birth control raises your risk. The evidence is somewhat reassuring. One study found that current oral contraceptive use was associated with a substantially lower risk of fibroids compared with never using them, and risk decreased further with longer duration of use.21PubMed. Use of oral contraceptives and uterine fibroids: results from a case-control study A separate study in a cohort of Black women found a modest inverse association between ever using combined oral contraceptives and fibroid incidence over about three years of follow-up.22PLoS ONE. Combined oral contraceptive utilization and uterine fibroid incidence: A prospective study in a cohort of African-American women Another study of women using hormonal contraception found that about 38% had fibroids, compared with nearly 60% among women not using hormonal contraception.23PubMed Central. Impact of Contraception on Uterine Fibroids
The picture is complicated, though, by the fact that women who already have symptomatic fibroids may be less likely to be prescribed oral contraceptives, which could artificially make the non-contraceptive group look worse. And former users seem to have about the same risk as women who never used oral contraceptives, which suggests the protective effect fades after stopping. Still, there is no good evidence that standard hormonal birth control triggers new fibroids.
The Gut Microbiome Connection
An emerging area of research explores whether the bacteria in your gut influence fibroid risk. Certain gut bacteria regulate the amount of circulating estrogen by producing enzymes that determine whether estrogen gets recycled back into the bloodstream or excreted. An imbalanced gut microbiome could theoretically shift that process toward higher estrogen levels. Researchers have proposed that gut bacteria may contribute to fibroids through estrogen metabolism, altered immune function, chronic inflammation, and changes in gut metabolites.24PubMed Central. The Gut Microbiota: a Novel Player in the Pathogenesis of Uterine Fibroids This research is still in its early stages, with most evidence coming from observational comparisons of gut bacteria in women with and without fibroids. No one has yet shown that deliberately changing the gut microbiome prevents or treats fibroids, but the hypothesis connects to the broader picture of how diet, body weight, and hormone levels all feed into fibroid risk.
What You Can and Cannot Control
Some risk factors for fibroids, like your genetics, race, or the age you got your first period, are not within your power to change. But others are. Maintaining a healthy body weight, staying physically active, eating a diet rich in fruits and vegetables, getting enough vitamin D, and minimizing unnecessary exposure to endocrine-disrupting chemicals in plastics and personal care products all have at least some support in the research literature. None of these steps guarantee prevention. A woman who does everything “right” can still develop fibroids, and many women with multiple risk factors never do. Fibroids remain common enough across all populations and lifestyles that no single cause fully explains them.
If you already have fibroids, the modifiable risk factors are still worth paying attention to. Weight loss and exercise have been associated with slower growth, and some treatments like selective progesterone receptor modulators work precisely by targeting the hormonal mechanisms described above. The fact that fibroids depend on a whole ecosystem of hormonal, metabolic, and environmental signals means there are multiple levers that treatment and lifestyle can pull, even if none of them is a guaranteed fix.