What Are Uterine Fibroids and What Causes Them to Grow?

Fibroids are benign growths or tumors of your uterus. They are very common and more than half of women will have fibroids by the age of 50. That being said, not all fibroids are symptomatic.

Another common medical term for uterine fibroids is leiomyoma or simply myoma. A popular slang term for fibroids is “fireballs”, which you will see is actually a really great description of what fibroids look like and the symptoms they can cause.

Mature female patient in exam room with doctor
Thomas Barwick / Getty Images

As I am sure you know, your uterus has one important function in your body and that is to allow you to carry a pregnancy. The lining of the uterus or endometrium changes to support the growing embryo while the wall of the uterus expands to accommodate the growing embryo and contracts at the onset of labor. The wall of your uterus can do these things because it is a muscle and that is what muscle does, expand and contract.

Understanding that your uterus is a muscle is important because uterine fibroids are tumors made up of muscle fibers. We don’t know exactly what causes fibroids to develop but we do know that they come from a single smooth muscle cell. What this means is that there is some trigger or triggers (still unknown to researchers) that cause a single smooth muscle cell in your uterus to duplicate and create a growth or tumor.

These altered smooth muscle fibers grow upon themselves creating a round mass with a whirled appearance. In fact, if you cut open a uterine fibroid it looks like the inside of a baseball.

What Causes Fibroids?

As just mentioned, the exact cause of fibroids is unknown.

 We do know that hormones and genetic factors likely play a role in the development of fibroids. Probably environmental factors contribute as well. We are starting to understand the impact of environmental toxins on our reproductive health and exposure to estrogen disrupting chemicals may contribute to the development of these tumors as well.

 We also know that there are certain factors that increase your risk of developing uterine fibroids including:

  • Age: Fibroids are most common between the ages of 40-50. They are uncommon before the age of 20. Fibroids will shrink after menopause.
  • Race: Fibroids are much more common in Black women. Again the reason why is not known. In addition to occurring more frequently, they often grow faster. Also, Black women may present with symptoms earlier often around age 30.
  • Obesity: Obesity probably increased the risk of fibroids, though more research is needed.
  • Family History: You are at an increased risk of developing fibroids if other women in your family have or have had fibroids.
  • Parity: There is an association between never being pregnant and developing fibroids

Types of Fibroids

There is only one “type” of fibroid but fibroids are further classified based on their location. Your healthcare provider may have used one of these terms:

  • Subserosal- The fibroid is in the outermost part of the uterine wall and distorts the shape of the uterus
  • Pedunculated- The fibroid developed in the outer wall of the uterus but has grown away from the uterus and is connected to the uterus by a stalk
  • Intramural- The fibroid is located in the wall of the uterus it usually does not distort the shape of the uterus
  • Transmural- The fibroid extends through the entire wall of the uterus and likely distorts the shape of the uterus
  • Submucosal- The fibroid developed in the innermost part of the uterus and grows into the lining of the uterus. This type of fibroid distorts the inside of the uterus or endometrial cavity.
  • Parasitic- A rare type of fibroid. It occurs when a fibroid that developed in the uterus detaches itself from the wall of the uterus and attaches somewhere else in your body.

The location and size of your fibroids will impact the symptoms you may be experiencing.

A Word From Verywell

 It is likely that you were diagnosed with fibroids because you complained to your doctor about pelvic pain or heavy periods. But, it is also possible that you were diagnosed with fibroids at a routine doctor's visit and are having no symptoms at all. Either way, be sure to discuss any questions or concerns you have with your doctor so you can live with uterine fibroids.

Was this page helpful?
Article Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. Uterine fibroids: MedlinePlus Medical Encyclopedia. MedlinePlus. Nov 6, 2019.

  2. Uterine fibroids. womenshealth.gov. Apr 1, 2019.

  3. Stewart EA, Cookson CL, Gandolfo RA, Schulze-rath R. Epidemiology of uterine fibroids: a systematic review. BJOG. 2017;124(10):1501-1512. doi:10.1111/1471-0528.14640

  4. Stewart EA, Nicholson WK, Bradley L, Borah BJ. The burden of uterine fibroids for African-American women: results of a national survey. J Womens Health (Larchmt). 2013;22(10):807-16. doi:10.1089/jwh.2013.4334

  5. Ilaria soave, Marci R. From obesity to uterine fibroids: an intricate network. Curr Med Res Opin. 2018;34(11):1877-1879. doi:10.1080/03007995.2018.1505606

Additional Reading
  • Stewart, E.A. (2015). Uterine fibroids. N Engl J Med, 372, 1646-1655. doi:10.1056/NEJMcp1411029