What causes uterine fibroids to grow?

Uterine fibroids, or leiomyomas, are benign tumors that arise from smooth muscle cells located in or around your uterus. The exact cause of fibroids is not known, but researchers believe the female sex hormones estrogen and progesterone that are produced by the ovaries contribute to the growth of the tumors.

Genetics may also play a role in causing uterine fibroids, but research is still ongoing.

Fibroids grow in or near the uterus. Their location can be:

• Submucosal, inside the uterus
• Intramural, within the uterine wall
• Subserosal, outside of the uterus

Some grow on branches that are connected to the inside or outside of the organ; these are called pedunculated fibroids.

All uterine fibroids are composed of muscle cells and fibrous connective tissue, similar to the uterus itself. Uterine fibroids have more estrogen receptors and progesterone receptors than the normal tissue that surrounds them, making them particularly sensitive to the hormones. Usually, fibroids grow much faster during pregnancy due to high levels of hormones.

While some fibroids can be so small they can only be detected microscopically, others can grow to be quite large, expanding to fill up the whole uterus and weighing several pounds.

The rate at which uterine fibroids grow depends on your specific case, as it is different with everyone and unpredictable. In some women, growth is very slow. In others, fibroids become larger more quickly. For example, during pregnancy when hormone levels are high, fibroids are known to grow faster. Treatment depends on your symptoms, not on the size of your fibroids.

Risk factors for uterine fibroids

Up to 80% of women will develop fibroids during their reproductive years. Uterine fibroids are very rare before puberty. The tumors usually occur in women between the ages of 20 and 50 when hormone levels are high or fluctuating, and they begin to shrink during menopause as hormones decline.

There are several risk factors associated with increased odds of developing uterine fibroids:

• It is known that women who are obese are more likely to develop fibroids because estrogen levels are higher if you are overweight. A large study in the European Journal of Epidemiology of more than 3,500 women showed that obesity nearly doubles the risk of uterine fibroids even after menopause due to increased estrogen produced by adipose tissue.
• Use of hormone therapy after menopause also doubled the risk of uterine fibroids after menopause at any weight.
• Additionally, Black women are at a slightly higher risk of developing uterine fibroids than white or Asian women.
• If you have a family history of uterine fibroids, you are also at increased risk, and your risk increases as you age.
• If you have a vitamin D deficiency, high blood pressure, you eat a lot of foods containing preservatives or you consume a lot of soybean products, your risk is higher as well.

On the flip side, your risk decreases if you have had children; in fact, the more children you have, the more your risk of developing uterine fibroids lowers.

Uterine fibroids treatment

Uterine fibroids typically become smaller when anti-hormone medications are used as treatment. Because uterine fibroids are responsive to the hormones estrogen and progesterone, treatment often involves hormone therapy such as gonadotropin-releasing hormone (GnRH) agonists or selective progesterone receptor modulators (SPRMs). These medications are most commonly used to shrink tumors prior to surgery or in women who are nearing menopause. Other hormonal treatment options may include oral contraceptives or a levonorgestrel-releasing intrauterine system (LNG-IUS).

Many times, fibroids shrink and go away even without treatment, so treatment may not be needed if you are asymptomatic or your symptoms are mild and not bothersome. This is especially true after menopause. Still, menopause does not completely prevent uterine fibroids from occurring.