No one knows exactly what causes fibroids. The leading explanation is that a fibroid starts when a single muscle cell in the wall of the uterus picks up a genetic change and begins to grow, and that the hormones estrogen and progesterone then fuel that growth. Your age, family history, race, weight, blood pressure and childbearing history all change your risk, but none of them alone causes fibroids.
How a fibroid starts
Fibroids, also called leiomyomas, are noncancerous growths of the smooth muscle of the uterus. Each fibroid is thought to grow from a single cell. A 2026 review describes them as "benign monoclonal tumors derived from myometrial stem or progenitor cells", which means every cell in a fibroid traces back to one starting cell in the uterine muscle.
Genetic changes inside the fibroid tissue appear to be the spark. In a 2011 study published in Science, researchers found changes in a gene called MED12 in 70% of fibroids tested (159 of 225 tumors from 80 women). These were changes found in the tumors, not genes the women were born with. The National Institute of Child Health and Human Development (NICHD) lists several other genes that may be involved as well.
ACOG's patient guidance adds that some research suggests fibroids may develop from misplaced cells that are present before birth. This is still being studied.
Hormones fuel fibroid growth
Once a fibroid exists, estrogen and progesterone help it grow. The federal Office on Women's Health (OWH) explains the pattern:
- Fibroids can grow quickly during pregnancy, when hormone levels are high.
- They shrink when medicines that block these hormones are used.
- They usually stop growing or shrink after menopause.
This is why fibroids are most often diagnosed during the reproductive years and why many women notice fewer symptoms after menopause. Read more in fibroids after menopause.
Birth control is a common question. Large reviews used by ACOG list oral contraceptives as linked with a lower risk of developing fibroids, and ACOG considers hormonal contraceptives a reasonable option for heavy bleeding with fibroids. ACOG's patient FAQ also notes that medicines containing estrogen may cause existing fibroids to grow. If you have fibroids and are choosing a contraceptive, talk it through with your gynecologist.
Questions about your treatment options? Dr. Fox can help.
Book a ConsultationThis article is part of our Fibroid Treatment Guide.
Risk factors and how strong the evidence is
A risk factor is something linked with a higher or lower chance of fibroids. Most of the evidence comes from observational studies, which show associations rather than proof of cause. The table sorts the main factors by evidence quality.
| Factor | Effect on risk | Strength of evidence |
|---|---|---|
| Age | Risk rises through the 30s and 40s until menopause | Strong (ACOG 2021; Stewart 2017 review) |
| Black race | About 2 to 3 times higher risk | Strong (Baird 2003 ultrasound study of 1,364 women; Stewart 2017) |
| Family history | Higher; OWH says about 3 times higher if your mother had fibroids | Consistent (ACOG 2021; Stewart 2017) |
| Obesity | Higher; OWH says 2 to 3 times higher in women with severe obesity | Moderate (ACOG 2021) |
| High blood pressure | Higher | Moderate (ACOG 2021; Stewart 2017) |
| Longer time since last birth | Higher | Moderate (ACOG 2021; Stewart 2017) |
| Having given birth (parity) | Lower; more births, lower risk | Consistent (ACOG 2021; Stewart 2017) |
| Oral contraceptives or the birth control shot | Lower | Moderate (ACOG 2021; Stewart 2017) |
| Low vitamin D | Higher; sufficient levels linked with 32% lower odds | Limited (one 2013 observational study) |
| Diet | Lots of red meat and ham linked with higher risk; green vegetables with lower | Limited (OWH; few studies) |
| Major stress | Possibly higher | Limited (listed by NICHD as under study) |
A closer look at the bigger risk factors
Age
Fibroids become more common with each decade until menopause. ACOG estimates they occur in up to 70% of women by menopause, although only about a quarter are significant enough to need treatment.
Race
In the Baird 2003 study, more than 80% of Black women and nearly 70% of white women had developed fibroids by age 50. Black women also tend to develop them earlier and have more severe symptoms. ACOG notes that racism and inequities in society may contribute. Our article on fibroids in Black women covers this in depth.
Family history
Fibroids run in families. If your mother or sister had them, mention it to your doctor, especially if you have heavy or painful periods.
Pregnancy and childbirth
Having given birth is one of the most consistent protective factors. ACOG's 2021 bulletin lists increasing parity, meaning more births, as lowering risk, and a longer time since your last birth as raising it. Researchers are still working out why. This is a pattern seen across groups of women, not a reason to plan a pregnancy around fibroid risk.
Weight and blood pressure
Obesity and high blood pressure are both listed by ACOG as risk factors. These links come from observational studies, so it is not proven that losing weight or lowering blood pressure will shrink fibroids you already have. Both are still worth managing for your heart and overall health.
Vitamin D
In a 2013 study by Baird and colleagues, women with vitamin D levels above 20 ng/mL had an estimated 32% lower odds of fibroids. Because the study was observational, it does not prove that taking vitamin D prevents fibroids. Ask your doctor before starting a supplement.
Can you prevent fibroids?
There is no proven way to prevent fibroids. Some factors, such as age, race and family history, cannot be changed. Keeping a healthy weight and healthy blood pressure are good for your overall health and are linked with lower fibroid risk, but they do not ensure you will never get fibroids. Many women with no risk factors develop fibroids, and many with several risk factors never have symptoms.
Why the cause matters less than your symptoms
Knowing your risk factors can help explain why fibroids showed up, but treatment decisions depend on your symptoms, the size and location of your fibroids, and your plans for pregnancy. Fibroids that cause no symptoms often need no treatment. If you have heavy bleeding, pressure or pain, effective options range from medication to procedures that keep the uterus, such as uterine fibroid embolization (UFE), and surgery. Start with what uterine fibroids are, or compare every option in our fibroid treatment guide.
Frequently Asked Questions
Are fibroids genetic?
Partly. Most fibroids carry genetic changes that arose in the fibroid itself, such as changes in the MED12 gene found in 70% of tumors in one 2011 study. Fibroids also run in families, so a family history raises your risk.
Can stress cause fibroids?
NICHD lists major stress among factors that might play a role, but the evidence is limited. Stress is not considered a direct cause.
Does birth control cause fibroids?
Large reviews link oral contraceptives with a lower risk of developing fibroids. ACOG notes that estrogen-containing medicines may make existing fibroids grow, so discuss your choice with your gynecologist if you already have fibroids.
Can diet cause fibroids?
The Office on Women's Health says eating a lot of red meat and ham is linked with higher risk and green vegetables with lower risk. These links come from a small number of observational studies, so diet is not a proven cause.
Did I do something to cause my fibroids?
No. Fibroids are very common and are driven mainly by genetic changes in the uterine muscle and normal hormones. Many of the known risk factors, such as age, race and family history, are outside your control.
Keep Reading
- Uterine Fibroid Embolization (UFE) in New York City
- Fibroids in Black Women: What the Research Shows and What You Can Do
- What Are Uterine Fibroids?
- Fibroids After Menopause: What Changes and What to Watch For
- Types of Uterine Fibroids: Submucosal, Intramural, Subserosal and the FIGO System
- Fibroid Medications: What Works, for How Long, and When to Consider a Procedure
- Fibroid Treatment Guide: every option, compared
Sources
- ACOG Practice Bulletin No. 228, Obstet Gynecol, 2021 — management of symptomatic uterine leiomyomas
- Stewart et al., BJOG, 2017 — epidemiology of uterine fibroids: a systematic review (60 publications)
- Mäkinen et al., Science, 2011 — MED12 is mutated at high frequency in uterine leiomyomas (225 tumors)
- Pecorella et al., Diagnostics, 2026 — adenomyosis and uterine fibroids: distinct yet overlapping entities
- Baird et al., Am J Obstet Gynecol, 2003 — cumulative incidence of fibroids by age 50 (1,364 women)
- Baird et al., Epidemiology, 2013 — vitamin D and the risk of uterine fibroids
- Office on Women's Health — Uterine fibroids (updated February 2025)
- NICHD — What causes uterine fibroids?
- ACOG patient FAQ — Uterine Fibroids (last reviewed December 2025)
Medically reviewed by David Fox, MD, FACS, RPVI. This article is general information, not a substitute for a medical evaluation.
Living with heavy periods, pelvic pressure or other fibroid symptoms? Schedule a consultation with Dr. David Fox at Fox Vein & Vascular in Manhattan to find out whether uterine fibroid embolization is right for you. Call (212) 362-3470 or request a UFE consultation online.
