Progesterone is one of the primary hormones driving fibroid growth. It promotes the multiplication of fibroid cells and stimulates the production of the structural tissue that makes fibroids firm and bulky.1PubMed Central. Progesterone Signaling and Uterine Fibroid Pathogenesis; Molecular Mechanisms and Potential Therapeutics For decades, estrogen got most of the blame for fibroid development, but accumulating evidence has recast progesterone as an equally important, and possibly more direct, contributor to tumor growth. That shift in understanding has opened the door to newer drug therapies that specifically target progesterone’s activity in fibroid tissue.
Why Progesterone Was Overlooked for So Long
Estrogen dominated the fibroid conversation for most of the twentieth century. Fibroids are rare before puberty, grow during reproductive years, and typically shrink after menopause, a pattern that maps neatly onto estrogen levels. Early animal studies reinforced the idea, and many of the first medical treatments for fibroids were designed to suppress estrogen. Progesterone, meanwhile, was regarded mainly as a pregnancy-support hormone, and some clinicians even prescribed progestins to treat fibroids on the assumption that they would counteract estrogen’s effects.
That thinking began to change when researchers noticed something awkward: fibroids tend to grow fastest during the luteal phase of the menstrual cycle, the stretch after ovulation when progesterone is at its peak. More telling, studies of fibroid tissue showed that cell division rates in fibroids are highest when progesterone levels are elevated, not when estrogen alone is dominant. Recent reviews now describe progesterone as playing a “crucial role” in fibroid development and growth, a substantial upgrade from its former supporting-cast status.2PubMed Central. Progesterone signaling in uterine fibroids: Molecular mechanisms and therapeutic opportunities
This does not mean estrogen is irrelevant. Estrogen primes fibroid tissue to respond to progesterone by increasing the number of progesterone receptors on fibroid cells. Think of it as estrogen setting the stage and progesterone directing the performance. Without estrogen, fibroid tissue has fewer docking sites for progesterone, which is one reason fibroids shrink after menopause when both hormones decline. But the cell proliferation itself, the actual multiplying of tumor cells, appears to be driven more directly by progesterone signaling.
What Progesterone Does Inside Fibroid Tissue
Progesterone affects fibroids in two main ways. First, it directly stimulates fibroid cells to divide and multiply. When progesterone binds to its receptor inside a fibroid cell, it switches on growth-promoting pathways that push the cell through its replication cycle. This cell proliferation is the most straightforward mechanism by which fibroids get larger over time.1PubMed Central. Progesterone Signaling and Uterine Fibroid Pathogenesis; Molecular Mechanisms and Potential Therapeutics
Second, progesterone stimulates the production of extracellular matrix, the dense scaffolding of collagen and other structural proteins that surrounds fibroid cells. If you have ever felt that a fibroid is hard or rubbery during an ultrasound or exam, that firmness comes largely from this matrix material. A fibroid is not just a ball of rapidly dividing cells; much of its bulk is structural tissue. Progesterone promotes the synthesis of this collagen-rich matrix, which means it contributes to fibroid size even beyond its effect on cell numbers.3PubMed. Progesterone signaling in uterine leiomyoma biology: Implications for potential targeted therapy
This two-pronged effect, more cells plus more structural scaffolding around each cell, helps explain why fibroids can grow surprisingly fast during certain periods. A fibroid that seems stable for months may enlarge noticeably within a single menstrual cycle or during early pregnancy, both times when progesterone exposure is high.
Why Fibroids Are More Sensitive to Progesterone Than Normal Tissue
A natural question is why progesterone drives growth specifically in fibroids rather than in the surrounding normal uterine muscle. Part of the answer lies in the receptors themselves. Fibroid tissue contains a higher concentration of progesterone receptors than the adjacent normal muscle tissue in the majority of cases. One study using both antibody staining and protein analysis found that progesterone receptor levels were higher in fibroids than in neighboring myometrium in eight out of eleven women tested. The difference appeared to be regulated after the gene was already read, meaning the fibroid cells were not necessarily making more receptor messenger molecules but were holding onto more of the finished receptor protein.4PubMed. Distribution of the A and B forms of the progesterone receptor messenger ribonucleic acid and protein in uterine leiomyomata and adjacent myometrium
There are two main forms of the progesterone receptor, commonly called PR-A and PR-B. In fibroids, PR-A consistently dominates over PR-B. This matters because the two forms can trigger different downstream effects: PR-A tends to suppress some of the anti-growth signals that PR-B can activate. The imbalance toward PR-A in fibroid cells may tilt the cellular response to progesterone in a growth-promoting direction, helping explain why the same hormone that supports a healthy uterine lining can simultaneously fuel abnormal tumor growth in fibroids.
The Stem Cell Connection
One of the more interesting discoveries in recent fibroid research involves stem cells. Fibroids contain a small population of stem-like cells that can self-renew and generate the mature tumor cells that make up the bulk of the growth. Here is the catch: these stem cells typically do not carry progesterone receptors themselves, or carry very few. So how does progesterone influence them?
The answer is indirect. Mature fibroid cells that do have progesterone receptors respond to the hormone and then send chemical signals to the nearby stem cells, essentially telling them to divide and produce more tumor cells. This relay system uses the WNT/β-catenin pathway, a well-known signaling route involved in cell growth across many tissue types.5PubMed. Uterine Leiomyoma Stem Cells: Linking Progesterone to Growth The practical implication is that progesterone’s growth-promoting influence extends to cells it cannot even directly contact, making it harder for the body to naturally rein in fibroid expansion once the signaling loop is established.
Progesterone, Life Stages, and Fibroid Behavior
Fibroids are extraordinarily common, with estimates suggesting a prevalence of up to 80 percent in premenopausal women, though many of those fibroids are small and cause no symptoms.1PubMed Central. Progesterone Signaling and Uterine Fibroid Pathogenesis; Molecular Mechanisms and Potential Therapeutics The behavior of fibroids tracks closely with the hormonal landscape at different life stages.
During reproductive years, progesterone levels rise and fall with each menstrual cycle. Fibroids may fluctuate slightly in size across a cycle, and many women notice that fibroid-related symptoms like pelvic pressure or heavy bleeding are worst during the luteal phase, when progesterone peaks. Over months and years, repeated exposure to these cyclical progesterone surges accumulates, and fibroids tend to grow gradually.
Pregnancy brings a sustained and dramatic rise in progesterone. Some fibroids grow substantially during pregnancy, particularly in the first trimester when progesterone is climbing steeply. Others remain stable or, paradoxically, may even shrink, likely because the blood supply dynamics and other hormonal shifts of pregnancy are complex. Clinicians monitor known fibroids during pregnancy because rapid growth can occasionally cause pain from tissue degeneration inside the fibroid or, in rare cases, complications with delivery.
After menopause, both estrogen and progesterone production from the ovaries drops sharply. This hormonal withdrawal is the main reason fibroids tend to shrink over time in postmenopausal women. Natural regression often begins in the menopausal transition itself.6PubMed Central. Uterine fibroids in menopause and perimenopause However, women who take hormone replacement therapy that includes a progestin may not see the same degree of shrinkage, and in some cases fibroids can remain stable or even grow slightly, because the exogenous progestin partially replaces the progesterone signal the fibroid cells were accustomed to.
Drugs That Target Progesterone Signaling
The recognition that progesterone is central to fibroid growth has led to a class of drugs called selective progesterone receptor modulators, or SPRMs. These compounds sit in the progesterone receptor and block or partially block progesterone’s signal. The most studied SPRM for fibroids is ulipristal acetate.
Clinical trials of ulipristal acetate showed substantial results. In one study of repeated 12-week treatment courses, roughly 62 to 73 percent of patients achieved complete cessation of menstrual bleeding, and more than 80 percent achieved controlled bleeding during both courses. After a second treatment course, fibroid volume shrank by a median of 54 to 58 percent, depending on the dose. Pain scores and quality of life improved, and fewer than 5 percent of patients stopped treatment because of side effects.7PubMed. Efficacy and safety of repeated use of ulipristal acetate in uterine fibroids Those numbers are striking for a pill-based treatment and illustrate how dependent fibroids are on progesterone signaling for their maintenance.
However, ulipristal’s story took a turn. Reports of serious liver injury, though rare, prompted regulatory reviews. The estimated risk of severe drug-induced liver injury sits around 1.5 per 100,000 users, and fatal liver injury at roughly 0.1 per 100,000.8PubMed Central. Risk–efficacy balance of ulipristal acetate compared to surgical alternatives These are low numbers in absolute terms, but they were enough for the European Medicines Agency to restrict its use and for the FDA to proceed cautiously. In some countries ulipristal acetate for fibroids is no longer available or requires liver function monitoring. The saga underscores a recurring theme in fibroid treatment: effective medical options exist, but they often come with trade-offs that force careful shared decision-making between patient and clinician.
Newer Combination Approaches
A different strategy for leveraging progesterone’s role in fibroids uses GnRH receptor antagonists combined with low-dose hormonal add-back. Relugolix combination therapy pairs a drug that suppresses the brain’s signal to produce reproductive hormones with small amounts of estrogen and a progestin. The antagonist sharply lowers estrogen and progesterone production, which starves the fibroid, while the add-back hormones are dosed just high enough to prevent the hot flashes and bone loss that would otherwise come with near-menopausal hormone levels.
In large trials, relugolix combination therapy reduced menstrual blood loss volume, improved pain, and decreased symptom severity compared to placebo. These benefits held regardless of where the largest fibroid was located in the uterus.9Obstetrics & Gynecology. Relugolix Combination Therapy Improves Uterine Fibroid Symptoms Independent of Fibroid Location Relugolix combination therapy received FDA approval in 2021 for management of heavy menstrual bleeding associated with fibroids in premenopausal women, making it one of the first oral options specifically approved for that indication.
The inclusion of a progestin in the add-back component might seem contradictory if progesterone fuels fibroids. The dose used in the add-back is far below what the ovaries would normally produce, and the net effect of suppressing the body’s own, much larger, progesterone output far outweighs the small amount replaced. The progestin’s job in the add-back is specifically to protect the uterine lining from the unopposed estrogen in the combination, not to treat fibroids directly.
The Progestin-Releasing IUD Paradox
If progesterone promotes fibroid growth, you might expect that a progestin-releasing intrauterine device would make things worse. In practice, the relationship is more nuanced. The levonorgestrel-releasing IUD delivers a synthetic progestin directly to the uterine lining but at very low systemic levels. In women with fibroids, this IUD reduces menstrual blood loss and likely reduces menstrual pain. However, it does not appear to reduce overall uterine dimensions or the size of the fibroids themselves.10PubMed. Progestin-releasing intrauterine systems and leiomyoma
This is an important distinction. The IUD’s benefit is symptomatic: it thins the uterine lining and reduces bleeding, which is the symptom that most disrupts daily life for many women with fibroids. But it does not attack the fibroid’s growth mechanism. For women whose main complaint is heavy periods and whose fibroids are not distorting the uterine cavity too much for the IUD to sit properly, it can be a practical option. For women whose symptoms stem from the fibroid’s size, such as pelvic pressure, urinary frequency, or pain from a large mass, the IUD is unlikely to help because the fibroid itself stays put.
This paradox sometimes confuses patients who read that progesterone drives fibroid growth and then learn their doctor is recommending a progestin-containing device. The explanation is dose and delivery: a localized low dose of progestin at the uterine lining is a different physiological situation than the cyclical whole-body progesterone surges produced by the ovaries each month.
MicroRNA Changes in Fibroid Tissue
Beyond the more familiar hormone-receptor-cell-growth story, researchers have found that fibroids differ from normal uterine muscle at the level of tiny regulatory molecules called microRNAs. These short stretches of genetic material act like volume dials, turning the activity of specific genes up or down. One study comparing fibroid tissue to normal adjacent muscle found 46 microRNA species that were expressed at different levels. Of those, 19 were overproduced and 27 were underproduced in fibroids, with changes ranging from just over 1-fold to nearly 12-fold.11Fertility and Sterility. Differential expression of microRNA species in human uterine leiomyoma versus normal myometrium
Some of these microRNAs sit in pathways that are influenced by progesterone, which raises the possibility that progesterone’s effects on fibroids extend further than the growth-promoting signals already described. Altered microRNA profiles could contribute to the fibroid’s ability to resist normal cell-death signals, accumulate excessive structural matrix, or maintain its own blood supply. This area of research is still young, and no therapies targeting microRNAs in fibroids exist yet. But it represents one reason scientists think the full picture of how progesterone shapes fibroid biology is more complex than a simple “hormone hits receptor, cell divides” model.
Practical Implications for Managing Fibroids
Understanding progesterone’s role changes the conversation you might have with your doctor in a few concrete ways. If you are being offered hormonal contraception and have known fibroids, the type of progestin, the dose, and the delivery method all matter. A systemic progestin at higher doses could theoretically support fibroid maintenance, while a localized low-dose device may help with bleeding without meaningfully feeding the fibroid. These are not decisions with universal right answers; they depend on fibroid size, location, symptoms, and your reproductive plans.
If you are approaching menopause and hoping fibroids will resolve on their own, progesterone’s decline is your ally. But if you plan to use hormone replacement therapy that includes a progestin, discuss whether your fibroid history warrants monitoring. Some women in this situation opt for estrogen-only therapy after hysterectomy, which removes the question entirely, but for those who still have a uterus, the progestin component is medically necessary to protect the endometrium, and the trade-off with fibroid stability deserves a frank conversation.
For women considering medical treatment to shrink fibroids before surgery or as an alternative to it, the newer drugs that target progesterone signaling, whether SPRMs or GnRH antagonist combinations, represent a meaningful expansion of options compared to a decade ago. They are not cures; fibroids typically regrow after treatment stops unless the underlying hormonal environment changes, as it does at menopause. But they can buy time, reduce symptoms, shrink tumors enough to permit less invasive surgery, or bridge the gap to menopause for women in their late forties who prefer to avoid an operation altogether.