Uterine fibroids are common, non-cancerous growths that develop within or on the uterus. While many individuals with fibroids experience no symptoms, others find these growths cause physical changes, such as the enlargement of the lower abdomen, which is sometimes mistaken for weight gain or pregnancy. This abdominal swelling is a direct consequence of the fibroids’ size and location, physically displacing and pushing against surrounding internal structures. Understanding the biology of these growths explains this change in body shape.
What Uterine Fibroids Are
Uterine fibroids are solid, benign tumors composed primarily of smooth muscle cells and fibrous connective tissue. They originate from a single, abnormal muscle cell within the uterine wall that divides repeatedly to form a firm, rubbery mass. These growths vary greatly in size, ranging from microscopic to masses larger than a grapefruit, and a person may have one or multiple fibroids simultaneously.
Fibroids are classified based on where they develop in the uterus. Intramural fibroids grow within the muscular wall of the uterus, often causing the entire organ to expand. Subserosal fibroids form on the outer surface and grow outward into the pelvic cavity. Submucosal fibroids grow just beneath the inner uterine lining, often protruding into the uterine cavity. Their growth is influenced by reproductive hormones, specifically estrogen and progesterone, which is why they typically shrink after menopause when these hormone levels decline.
The Physical Mechanism of Abdominal Enlargement
Abdominal enlargement is caused by mass effect, where the volume of the growths physically takes up space within the abdominal cavity. As fibroids grow, they increase the overall size of the uterus, which is normally about the size of a small pear. Large or numerous fibroids can cause the uterus to expand dramatically, sometimes reaching the size equivalent to a five- to seven-month gestation or a full-term pregnancy.
This increase in mass physically displaces the intestines and other abdominal contents, pushing the lower abdominal wall outward. Subserosal fibroids are particularly likely to cause a bulge because they grow on the outside of the uterus, expanding directly into the pelvic and abdominal space. Pedunculated fibroids, which are attached by a stalk, also contribute to this outward pressure and visible distention.
The firmness of the fibroid tissue, which is denser than surrounding fat or fluid, means the abdomen feels taut and full. This physical presence can be felt during a routine pelvic exam as a firm, irregular lump, confirming the mass effect is not due to bloating or fluid retention alone. When the total volume of the fibroids becomes substantial, it creates a constant, non-resolving protrusion.
Related Symptoms Caused by Size and Pressure
The physical displacement caused by large fibroids extends beyond visible abdominal enlargement, leading to secondary symptoms due to pressure on adjacent organs. When fibroids press on the bladder, which sits in front of the uterus, the bladder’s capacity is reduced. This mechanical compression results in increased urgency and frequency of urination, sometimes causing a person to wake up multiple times at night.
Fibroids positioned toward the back of the uterus can exert pressure on the rectum and colon. This pressure interferes with the normal movement of the bowels, leading to chronic constipation or a feeling of incomplete emptying. The mass effect can also compress nerves and blood vessels in the pelvis, which may cause referred pain, manifesting as chronic aches in the lower back or pain that radiates down one or both legs.
When to Consult a Healthcare Provider
Consulting a healthcare provider is important if abdominal enlargement is persistent or if new symptoms develop. Diagnosis typically involves a pelvic exam, where the provider can feel the enlarged uterus or firm masses, followed by an imaging test. The most common diagnostic tool is a pelvic ultrasound, which uses sound waves to visualize the uterus and any growths.
Immediate medical guidance is necessary if you experience sudden, sharp, and severe abdominal pain not relieved by typical medication. This acute pain could indicate a rare complication, such as the fibroid outgrowing its blood supply or a pedunculated fibroid twisting on its stalk. Confirming the cause of the enlargement helps determine the appropriate management, which may range from monitoring to considering interventions if symptoms significantly impact daily life.