Uterine fibroids, also known as leiomyomas, are benign growths that develop from the smooth muscle tissue of the uterus. They originate in the myometrium, the muscular wall of the uterus, and are the most common type of non-cancerous tumor in the female reproductive system. Their size can range from tiny seedlings to large masses that distort the uterus.
The Four Primary Classifications
Fibroids are classified based on their specific location within the uterine structure. The most common type is the intramural fibroid, which develops entirely within the myometrium, the thick, middle muscular layer of the uterine wall. As they expand, intramural fibroids can enlarge the overall size and shape of the uterus.
Subserosal fibroids grow on the outside surface of the uterus, just beneath the serosa, the thin outer layer. These growths project outward into the pelvic cavity, often remaining asymptomatic until they reach a considerable size.
Submucosal fibroids form just under the endometrium, the inner lining of the uterus, and protrude directly into the uterine cavity. Although they are the least common type, their placement can cause the most significant symptoms by interfering with the normal function of the uterine lining.
A pedunculated fibroid is a variation of either a subserosal or a submucosal tumor. This type is characterized by its attachment to the uterine wall by a slender, stem-like structure called a stalk. The stalk can form either on the outside of the uterus (pedunculated subserosal) or on the inside (pedunculated submucosal).
How Location Influences Common Symptoms
The anatomical position of a fibroid strongly dictates the types of symptoms a person may experience. Submucosal fibroids are intimately linked to heavy menstrual bleeding (menorrhagia) because their presence distorts the uterine cavity and disrupts the blood vessels of the endometrium, leading to prolonged and excessive blood loss.
This heavy bleeding can deplete the body’s iron stores, frequently resulting in iron-deficiency anemia and chronic fatigue. Submucosal growths can also interfere with reproductive health by blocking the fallopian tubes or preventing a fertilized egg from successfully implanting into the uterine wall.
Conversely, intramural and subserosal fibroids are more often associated with “bulk symptoms” as they grow larger. These tumors press upon adjacent organs, leading to a feeling of pelvic pressure or fullness. For example, a large subserosal fibroid positioned toward the front of the uterus can compress the bladder, causing a frequent or urgent need to urinate.
When a fibroid is located toward the back of the uterus, it can push against the rectum, contributing to issues like constipation. Pedunculated fibroids carry a specific risk distinct from bulk pressure. The stalk connecting the fibroid to the uterus can sometimes twist, a condition known as torsion, which can cause episodes of sudden and severe acute pain.
Hormonal and Genetic Risk Factors
The growth of uterine fibroids is primarily driven by the female reproductive hormones, estrogen and progesterone. Fibroid cells contain a higher number of receptors for these hormones, classifying them as hormone-dependent tumors. This dependence explains why fibroids are mainly a concern during the reproductive years.
Following menopause, when levels of these ovarian hormones naturally decline, fibroids typically shrink in size and symptoms often diminish. Beyond hormonal influence, a strong genetic component is a significant risk factor for fibroid development. Individuals whose mother or sister had fibroids face a markedly higher likelihood of developing them as well.
Specific genetic mutations, such as those in the MED12 gene, have been identified in a substantial number of fibroids, suggesting an inherent cellular abnormality that promotes uncontrolled proliferation. Age is also a factor, with prevalence increasing as women move through their reproductive years.
The prevalence and severity of fibroids also vary significantly by race, with Black women experiencing a higher incidence, an earlier age of onset, and often more numerous or larger tumors compared to women of other racial groups. Other non-genetic factors, including obesity, are thought to contribute to fibroid risk, potentially by influencing hormone metabolism.