Uterine leiomyoma, commonly known as fibroids, are noncancerous growths that develop in the muscular wall of the uterus. These tumors are remarkably common, affecting millions of women worldwide, yet many remain unaware they have them until discovered during routine medical examinations or imaging tests performed for other reasons.

Understanding Uterine Leiomyoma

Uterine leiomyomas are benign tumors, meaning they are not cancerous, that form from the smooth muscle cells and connective tissue of the uterus. The term “benign” indicates these growths do not have the ability to spread to other parts of the body or become cancer. In fact, the transformation of a fibroid into a cancerous growth occurs in less than one percent of cases, making this possibility extremely rare.^[7]

These growths can vary dramatically in size. Some fibroids are so tiny that they can only be seen under a microscope, while others can grow to the size of a grapefruit or even larger, sometimes filling the entire pelvis or abdomen. In extreme situations, very large fibroids can cause a woman’s belly to expand to the point where she may appear pregnant.^[1] Fibroids can appear as a single growth or in clusters, and it is quite common for women to have multiple fibroids at the same time.^[2]

The location where fibroids grow in the uterus varies, and this location often determines what symptoms, if any, a woman might experience. Intramural fibroids are embedded within the muscular wall of the uterus and are the most common type. Submucosal fibroids grow beneath the inner lining of the uterus and protrude into the uterine cavity. Subserosal fibroids develop on the outer surface of the uterus and can extend into the pelvis. Some fibroids attach to the uterus with a stalk-like structure, earning them the name pedunculated fibroids because they resemble mushrooms with a stem and wider top.^[2]

How Common Are Uterine Fibroids?

Uterine leiomyomas are extraordinarily common among women. Research indicates that between fifty and seventy percent of all women will develop at least one fibroid by the time they reach menopause. When looking at women by age fifty, studies show that up to eighty percent may have fibroids.^[3][4] This makes fibroids the most common type of noncancerous tumor found in women of reproductive age.^[2]

The prevalence of fibroids increases with age throughout a woman’s reproductive years. They are rare before a girl’s first menstrual period and become less common after menopause, when hormone levels naturally decline. Most fibroids are diagnosed in women between the ages of thirty and fifty, though they can occur at any point after menstruation begins.^[2][5]

Certain groups of women face higher rates of fibroid development. Women of African descent, particularly Black women, experience fibroids more frequently and at younger ages compared to women of other racial and ethnic backgrounds. In the United States, the prevalence among Black women reaches over eighty percent by age fifty, compared to approximately seventy percent in White women.^[3][7] Additionally, Black women often develop more severe symptoms that can significantly impact their daily lives and overall quality of life.^[4]

What Causes Uterine Fibroids?

The exact cause of uterine fibroids remains unknown, despite decades of research into these common growths. Scientists have not identified a single definitive reason why some women develop fibroids while others do not. However, what is well understood is that fibroids are highly responsive to hormones, particularly the female reproductive hormones estrogen and progesterone.^[4][5]

Fibroids originate from the smooth muscle cells of the uterus’s middle layer, called the myometrium. These tumors grow primarily under the influence of circulating estrogen and progesterone in the bloodstream. Fibroid tissue contains special receptors that respond to these hormones, and the tissue can even produce its own estrogen through a process involving an enzyme called aromatase.^[4] This hormone dependency explains why fibroids typically develop during a woman’s reproductive years when estrogen levels are highest, and why they often shrink after menopause when hormone production decreases dramatically.^[5]

Genetic factors also appear to play a role in fibroid development. Research suggests that these growths may run in families, indicating a hereditary component. Each fibroid is thought to arise from a single abnormal muscle cell that divides and multiplies to create a firm, rubbery mass.^[6]

Risk Factors for Developing Fibroids

While fibroids can develop in any woman of reproductive age, certain factors increase the likelihood of developing these growths. Understanding these risk factors can help women and their healthcare providers remain vigilant about screening and early detection.

Age represents one of the most significant risk factors. The risk of developing fibroids increases as women progress through their reproductive years, with the highest rates occurring in women over age forty until menopause begins.^[4] After menopause, when estrogen levels drop significantly, the risk decreases substantially and existing fibroids often shrink.^[6]

Race and ethnicity play a notable role in fibroid risk. As mentioned earlier, women of African or African-Caribbean descent face significantly higher rates of fibroids compared to women of White, Hispanic, or Asian backgrounds. These women also tend to develop fibroids at younger ages and experience more severe symptoms.^[4][5]

A family history of fibroids increases a woman’s risk, suggesting that inherited genetic factors contribute to their development. Women whose mothers or sisters have had fibroids are more likely to develop them as well.^[4]

Early onset of menstruation, particularly before age ten, is associated with increased fibroid risk. Conversely, women who experience their first period later in adolescence, especially after age sixteen, appear to have a lower risk.^[4] Body weight also influences risk, as being overweight or obese increases estrogen levels in the body, which may promote fibroid growth. Women with higher body weight face elevated fibroid risk compared to those maintaining a healthy weight.^[4][5]

Pregnancy and childbearing appear to offer some protection against fibroids. Women who have never been pregnant face higher risk, while those who have given birth multiple times show lower rates of fibroid development. Each pregnancy and birth seems to reduce the risk further.^[4] Interestingly, smoking has been associated with a decreased risk of fibroids, though this does not make smoking a recommended prevention strategy given its numerous serious health risks.^[4]

Common Symptoms of Uterine Fibroids

Many women with uterine fibroids never experience any symptoms and remain completely unaware of their presence. These asymptomatic fibroids are often discovered incidentally during routine pelvic examinations or imaging tests performed for other medical reasons. However, for the twenty to thirty percent of women who do experience symptoms, fibroids can significantly impact daily life and well-being.^[3]

The most common symptom of fibroids is abnormal uterine bleeding, particularly heavy menstrual bleeding. Women may experience periods that are exceptionally heavy, sometimes with large blood clots, and periods that last longer than usual. Some women also experience bleeding between their regular menstrual periods.^[2][4] This excessive bleeding can lead to anemia, a condition where the blood lacks adequate healthy red blood cells to carry oxygen throughout the body, causing fatigue and weakness.^[6]

Pelvic pain and pressure represent another common complaint. As fibroids grow, they can create a sensation of fullness or pressure in the lower abdomen or pelvis. Some women describe feeling bloated or experiencing discomfort similar to the feeling of being full after a large meal.^[2] The location and size of fibroids influence which symptoms appear and their severity.^[4]

Large fibroids can press against nearby organs, causing additional problems. When fibroids push against the bladder, women may need to urinate frequently or urgently. In some cases, fibroids can make it difficult to empty the bladder completely.^[2] Pressure on the rectum can lead to constipation or a feeling of pressure during bowel movements. Back pain, particularly in the lower back, can occur when fibroids press on nerves or muscles in the pelvic region.^[2][4]

Sexual function can be affected as well. Some women with fibroids experience pain or discomfort during sexual intercourse, which can impact intimate relationships and quality of life.^[2] Additionally, some women notice chronic vaginal discharge or find that their abdomen becomes visibly enlarged, sometimes to the point where they appear pregnant even when they are not.^[2]

The symptoms typically stabilize or disappear after menopause, as declining hormone levels cause most fibroids to shrink naturally. Some research has also shown that certain small fibroids may shrink on their own even in premenopausal women.^[6]

Prevention Strategies

Because the exact cause of uterine fibroids remains unknown, there are no guaranteed methods to prevent their development entirely. However, certain lifestyle choices and habits may help reduce the risk of developing fibroids or slow their growth once they appear.

Maintaining a healthy body weight through balanced nutrition and regular physical activity appears beneficial. Studies have found that women who maintain a healthy weight have lower rates of fibroid development compared to those who are overweight or obese. The connection likely relates to estrogen levels, as excess body fat increases estrogen production, which can promote fibroid growth.^[4][5]

Diet plays an important role in fibroid prevention. Research suggests that eating plenty of fruits and vegetables, particularly cruciferous vegetables like broccoli and cabbage, and fruits like apples and tomatoes, may lower the risk of developing fibroids. Diets rich in organic produce and whole grains appear more beneficial than those heavy in processed foods, red meat, and high-fat dairy products.^[4]

Regular physical exercise offers protective benefits. One study found that women who engaged in approximately seven hours per week of activities such as running, dancing, or walking had the lowest chance of developing fibroids. Exercise helps maintain healthy body weight, regulates hormone levels, and reduces blood pressure, all of which may contribute to lower fibroid risk.^[4]

Managing blood pressure is another important consideration. Research has demonstrated a strong association between high blood pressure and fibroids. Women can work with their healthcare providers to maintain healthy blood pressure through diet, lifestyle modifications, or medication when necessary.^[4] Some evidence suggests that certain supplements, particularly vitamin D, might help lower fibroid risk, though women should always discuss supplementation with their healthcare providers before beginning any new regimen.^[4]

How Fibroids Affect the Body

Understanding the physical and biochemical changes that occur when fibroids develop helps explain why these growths cause the symptoms they do. At the cellular level, fibroids represent an abnormal proliferation of smooth muscle cells from the uterine wall. These cells multiply excessively, forming firm, round masses of muscle and fibrous connective tissue.^[3]

Fibroids contain large amounts of extracellular matrix, a complex network of proteins and other molecules that provide structural support to cells. This matrix includes substances like collagen, which gives fibroids their characteristic firm, rubbery texture.^[10] A thin covering called a pseudocapsule, made of compressed muscle fibers and loose connective tissue, typically surrounds each fibroid, creating a boundary between the tumor and normal uterine tissue.^[3]

The hormonal influence on fibroids operates through specific pathways. Fibroid cells contain receptors that bind to estrogen and progesterone, the two primary female reproductive hormones. When these hormones attach to their receptors, they trigger signals that promote cell growth and division. Additionally, fibroid tissue produces local growth factors that further stimulate cell multiplication. The enzyme aromatase present in fibroid tissue can convert other hormones into estrogen right within the tumor, creating a self-sustaining cycle of hormone production and growth.^[4]

As fibroids grow, they can physically distort the normal shape and structure of the uterus. Depending on their location, they may bulge into the uterine cavity where a pregnancy would develop, protrude outward from the uterine surface into the pelvic cavity, or remain embedded within the muscular wall. This physical distortion explains many of the mechanical symptoms women experience.^[1]

When fibroids cause heavy menstrual bleeding, the mechanism involves both mechanical and hormonal factors. Submucosal fibroids that project into the uterine cavity increase the surface area of the endometrium, the lining that sheds during menstruation. This larger surface means more tissue to shed and consequently heavier bleeding. Fibroids can also interfere with the normal contraction of uterine muscles that helps control blood flow during menstruation.^[10]

The pressure symptoms result from simple mechanical compression. As fibroids enlarge, they occupy space in the pelvis and can press against the bladder, causing frequent urination or urgency. Pressure on the rectum leads to constipation or difficult bowel movements. Large fibroids pushing against nerves in the pelvis or lower back can cause pain that radiates to these areas.^[2]

Fibroids can also impact fertility and pregnancy, though the mechanisms are complex and not fully understood. They may interfere with sperm transport through the uterus, disrupt implantation of a fertilized egg, or compete with a developing pregnancy for blood supply and space. During pregnancy, fibroids may increase the risk of complications such as preterm labor, abnormal positioning of the placenta, or the need for cesarean delivery.^[3]