Uterine fibroids grow because of two things working together: hormones, mainly estrogen and progesterone, and genetic changes inside the muscle cells of the uterus. Estrogen drives their growth, progesterone helps maintain them, and gene mutations in those cells cause them to multiply when normal uterine muscle cells would not. Neither factor works alone. Take away the hormonal signal and fibroids usually shrink. Change the genes and the cells keep growing even when they should not.
This is why fibroids behave the way they do. They grow during the reproductive years, when hormone levels are high. They often shrink after menopause, when hormone levels fall. And they can run in families, because some of the genetic changes that drive them are inherited or shared within families.
Understanding this two-part mechanism explains a lot about who gets fibroids, why they grow at certain times, and why treatments that block hormones work the way they do.
What Causes Uterine Fibroids To Grow Hormones Genes?
Fibroids are benign tumors made of smooth muscle and connective tissue. They start from a single cell in the uterus that develops a genetic change, then begins dividing when it should not. That single cell becomes a small cluster, and hormones give it the fuel to keep expanding.
Estrogen is the primary driver. Uterine tissue is estrogen-sensitive, and fibroid cells carry more estrogen receptors than normal uterine muscle cells. That means they respond more strongly to the same hormone signal. Progesterone also plays a role. It appears to support the growth and survival of fibroid cells, and it may help explain why fibroids keep growing during the second half of the menstrual cycle.
Growth hormone and other signaling molecules add to the picture. But estrogen and progesterone remain the central hormonal forces.
The genetic side is just as important. Research has found that fibroid cells often carry mutations in genes that control how fast a cell divides. One gene that has been studied extensively is MED12. Mutations in this gene are found in a large share of fibroids, though not all of them. Other genes involved in chromosome structure and cell signaling have also been implicated.
Here is the key point: these mutations happen inside the fibroid cells themselves. They are not inherited mutations passed from parent to child in the way some cancer genes are. They are acquired changes, meaning they develop during a person’s lifetime. This is why a woman with fibroids does not necessarily pass the exact fibroid to her daughter — but she may pass a tendency toward developing them.
Why Do Fibroids Grow During The Reproductive Years?
Fibroids need estrogen to grow, and the reproductive years are when estrogen levels are at their highest. This is the simplest explanation for the timing pattern doctors see.
Fibroids are rare before puberty. They appear most often during the 30s and 40s, when women have been exposed to years of cyclical hormone surges. They can grow during pregnancy, when estrogen and progesterone rise sharply. And they usually shrink after menopause, when the ovaries stop producing estrogen.
This pattern is not a coincidence. It is a direct reflection of hormone dependence. When the hormonal signal is strong, fibroids grow. When it fades, they regress.
Obesity can raise estrogen levels because fat tissue converts other hormones into estrogen. This is one reason obesity is associated with a higher risk of fibroids. The same logic applies to early menstruation, which extends the number of years a woman is exposed to cycling hormones.
None of this means hormones are the only factor. It means they are a necessary one. Without estrogen and progesterone, fibroids typically do not grow, regardless of what genes are involved.
How Do Genes Make Fibroids Grow?
Genes control how cells divide, repair themselves, and respond to signals. When a gene that regulates cell division is altered, the cell can keep dividing without the normal brakes.
In fibroids, several genetic changes have been identified. The most studied is MED12, which helps regulate gene activity. Mutations in MED12 are found in a substantial portion of fibroids, though the exact percentage varies across studies. Other genes, including HMGA2 and FH, have been linked to fibroid development in certain cases.
These mutations are not the same in every fibroid. Two fibroids in the same uterus can have different genetic profiles. This is one reason fibroids can behave differently — some grow quickly, others stay small for years.
What the genetics explain is why fibroids form at all. A normal uterine muscle cell would not keep dividing beyond what the body needs. A cell with a mutation in a growth-control gene can. Hormones then determine how fast that abnormal growth proceeds.
Genetics also explain why fibroids run in families. A woman whose mother or sister had fibroids is more likely to develop them herself. This does not mean a specific gene for fibroids has been identified in the way some disease genes have. It means that inherited variations can raise susceptibility, and shared environmental factors may add to that risk.
Is Race A Factor In Fibroid Growth?
Fibroids are more common and often more severe in Black women compared to white women. Black women tend to develop fibroids at a younger age, have larger fibroids, and experience more symptoms.
The reasons are not fully understood. Hormonal differences, genetic factors, and environmental exposures have all been proposed. Some research suggests that Black women may have higher estrogen levels or different patterns of hormone metabolism. Others point to differences in vitamin D levels, which have been studied as a possible factor in fibroid development.
What is clear is that the disparity is real and well documented. It is not explained by access to care alone, though disparities in diagnosis and treatment certainly exist. The biological factors are still being investigated.
This matters for anyone trying to understand their own risk. A family history of fibroids, especially in a first-degree relative, is a meaningful risk factor. So is being Black. Neither is a guarantee, but both raise the likelihood.
What Does This Mean For Treatment?
If fibroids depend on hormones and genes, then treatments that target either one can work.
Hormonal treatments aim to reduce estrogen or block its effects. Gonadotropin-releasing hormone (GnRH) agonists, for example, put the body into a temporary menopausal state, lowering estrogen and shrinking fibroids. These are often used before surgery to reduce fibroid size. They are not a permanent solution because hormone levels return when the medication stops.
Other hormonal options include progestin-releasing intrauterine devices, which can reduce bleeding but do not necessarily shrink fibroids. Some medications target progesterone receptors directly. The evidence for how well each option works varies, and response differs from person to person.
Surgical treatments remove the fibroids themselves. Myomectomy removes fibroids while leaving the uterus intact. Hysterectomy removes the uterus entirely and is the only definitive cure. These procedures do not address the underlying genetic changes, so new fibroids can form after myomectomy.
Non-hormonal options exist as well. Tranexamic acid reduces bleeding but does not shrink fibroids. NSAIDs can help with pain but do not affect growth.
No treatment currently targets the genetic mutations directly. That is an area of active research, but no approved therapy exists yet.
Can Fibroids Be Prevented?
There is no proven way to prevent fibroids. Because the exact cause involves both genetic changes and hormonal exposure, prevention is difficult to study and even harder to achieve.
Some lifestyle factors are associated with lower risk. Maintaining a healthy weight may help, since obesity raises estrogen levels. Eating more fruits and vegetables and less red meat has been linked to lower fibroid risk in some studies, though the evidence is not strong enough to call it a prevention strategy. Vitamin D has been studied, but results are mixed and no clinical guideline recommends it for fibroid prevention.
What is clear is that fibroids are common. Most women will develop them at some point, though many will never have symptoms. For those who do, the goal is management, not prevention.
If you have symptoms — heavy bleeding, pelvic pain, pressure, frequent urination — see a doctor. Fibroids are not dangerous in themselves, but they can cause significant discomfort and complications. The treatment approach depends on your symptoms, your age, and whether you want to have children.
Frequently Asked Questions
Do fibroids always grow during pregnancy?
Not always, but they often do because estrogen and progesterone rise sharply during pregnancy. Some fibroids stay the same size, and a few may even shrink.
Can fibroids turn into cancer?
No. Fibroids are benign and do not become cancerous. A rare cancer called leiomyosarcoma can look similar on imaging, but it does not develop from a fibroid.
Do fibroids go away after menopause?
They often shrink after menopause because estrogen levels drop. They do not always disappear completely, and some may remain the same size.
Are fibroids hereditary?
Having a mother or sister with fibroids raises your risk, so there is a familial pattern. But no single inherited gene has been identified as the cause.
