Why do uterine fibroids form

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Last updated: April 8, 2026

Quick Answer: Uterine fibroids form from the abnormal growth of smooth muscle cells and connective tissue in the uterus, influenced by hormonal factors like estrogen and progesterone. They affect 20-80% of women by age 50, with higher prevalence in Black women (up to 80%) compared to White women (about 70%). Most develop during reproductive years, typically between ages 30-40, and can vary in size from seedlings to large masses that distort the uterus. While often asymptomatic, they cause symptoms like heavy bleeding or pelvic pain in about 25% of cases.

Key Facts

Affect 20-80% of women by age 50, with higher rates in Black women (up to 80%)
Most common in reproductive years, typically ages 30-40
Influenced by hormones like estrogen and progesterone
Can range from seedlings to large masses distorting the uterus
Cause symptoms in about 25% of cases, such as heavy bleeding or pain

Overview

Uterine fibroids, also known as leiomyomas or myomas, are noncancerous growths that develop in the muscular wall of the uterus. They are among the most common gynecological conditions, with studies showing they affect a significant portion of women worldwide. Historically, fibroids have been documented since ancient times, with descriptions found in Egyptian medical texts from around 1500 BCE. In modern medicine, their prevalence varies by population, but they are particularly common in women of reproductive age, with incidence peaking in the 30s and 40s. The condition has been extensively studied since the 19th century, with surgical treatments like myomectomy and hysterectomy evolving over time. Today, fibroids are a major cause of gynecological hospitalizations, accounting for over 200,000 hysterectomies annually in the U.S. alone, highlighting their ongoing clinical significance.

How It Works

Uterine fibroids form through a complex process involving the abnormal proliferation of smooth muscle cells and fibroblasts in the myometrium, the uterine muscle layer. The exact cause is not fully understood, but hormonal factors play a key role: estrogen and progesterone stimulate fibroid growth, as evidenced by their tendency to shrink after menopause when hormone levels drop. Genetic factors also contribute, with mutations in genes like MED12 found in up to 70% of fibroids. Growth factors such as TGF-β and IGF-1 promote cell division and extracellular matrix production, leading to firm, rubbery tumors. Fibroids can develop as single nodules or clusters, ranging from small seedlings to large masses that distort the uterine cavity. They are classified by location: submucosal (under the lining), intramural (within the wall), or subserosal (on the outer surface). This localization affects symptoms and treatment options, with submucosal fibroids often linked to heavy menstrual bleeding.

Why It Matters

Uterine fibroids matter because they significantly impact women's health and quality of life, especially during reproductive years. They are a leading cause of heavy menstrual bleeding, which can lead to anemia and fatigue, affecting daily activities. Fibroids also contribute to pelvic pain, pressure symptoms like frequent urination, and reproductive issues such as infertility or pregnancy complications like miscarriage or preterm labor. Economically, they result in substantial healthcare costs, with estimates of billions spent annually on treatments and lost productivity in the U.S. The condition underscores disparities in healthcare, as Black women experience higher prevalence, severity, and earlier onset, highlighting needs for equitable access to care. Understanding fibroid formation drives research into less invasive treatments, such as MRI-guided focused ultrasound, improving outcomes for millions globally.