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what are fibroids

What Are Fibroids?

Fibroids are non-cancerous tumors of the uterus

"What are fibroids" is a common question when first learning that you or a loved one might have them. Uterine fibroids (also called ‘leiomyomas’) are extremely common benign (non-cancerous) smooth muscle tumors of the uterus. They arise in reproductive-age women and typically present with symptoms of abnormal uterine bleeding, pelvic pain, and/or pelvic pressure.

Fibroids can develop at various locations throughout the uterus at sizes ranging from undetectable seedlings to massive tumors that distort the shape of the uterus and obstruct nearby organ systems.¹ Fibroids receive the majority of their blood supply directly from the uterine arteries² and have a tendency to grow over time if left untreated.¹

Layers of the uterus

The healthy uterus is made up of three primary layers of tissue: the endometrium, the myometrium, and the perimetrium. The endometrium (inside layer) is the mucous membrane that lines the inside of the uterus. The endometrium thickens during the menstrual cycle and sheds to cause healthy bleeding during menstruation. The myometrium (middle layer) is composed primarily of smooth muscle cells (hence the prefix myo) whose primary function is uterine contraction - a normal physiological phenomenon occuring during the menstrual cycle, often termed “menstrual cramps”. The perimetrium (outer layer) lines the outside of the uterus.

A look at what fibroids are.

Where do fibroids form?

Uterine fibroids are monoclonal tumors (originating from a single “bad” cell) that arise from uterine smooth muscle cells (myometrium). They can grow within the myometrium, on the outside of the uterus, just underneath the endometrium, or on small stalks inside or outside the uterus.


Clinically significant fibroids on the outside of the uterus can press on nearby organs such as the bladder or the gastrointestinal tract, causing sensations of pressure in the pelvis, frequent urination, difficulty urinating, or constipation. Alternatively, fibroids just under the inner lining of the uterus can cause heavy bleeding and contribute to iron-deficiency anemia. Because they inhabit the uterus - altering its shape and function - fibroids may also contribute to menstrual cycle abnormalities, pain during intercourse, and pregnancy challenges.³⁻⁶

Check out more fibroid facts and read our post on the differences between a fibroid, cyst, and polyp to learn more.


[1] Pelage, J.-P., Cazejust, J., Pluot, E., Le Dref, O., Laurent, A., Spies, J. B., … Lacombe, P. (2005). Uterine Fibroid Vascularization and Clinical Relevance to Uterine Fibroid Embolization. Radiographics, 25, S99–S117.

[2] Peddada, S. D., Laughlin, S. K., Miner, K., Guyon, J.-P., Haneke, K., Vahdat, H. L., … Baird, D. D. (2008). Growth of uterine leiomyomata among premenopausal black and white women. Proceedings of the National Academy of Sciences, 105(50), 19887–19892.

[3] Khan, A. T., Shehmar, M., Gupta, J. K., & Gupta, J. (2014). Uterine fibroids: current perspectives. International Journal of Women’s Health, 6, 95–114.

[4] Zimmermann, A., Bernuit, D., Gerlinger, C., Schaefers, M., & Geppert, K. (2012). Prevalence, symptoms and management of uterine fibroids: An international internet-based survey of 21,746 women. BMC Women’s Health, 12(1), 6.

[5] Gupta, S., Jose, J., & Manyonda, I. (2008). Clinical presentation of fibroids. Best Practice and Research: Clinical Obstetrics and Gynaecology, 22(4), 615–626.

[6] Duhan, N., & Sirohiwal, D. (2010). Uterine myomas revisited. European Journal of Obstetrics Gynecology and Reproductive Biology, 152(2), 119–125.

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