Uterine fibroids, also known as uterine leiomyomas, fibromyoma or fibroids, are benign smooth muscle tumors of the uterus, part of the female reproductive system. Most women with fibroids have no symptoms, while others may have painful or heavy menstrual bleeding. If large enough, fibroids may push on the bladder and cause frequent need to urinate. They may also cause pain during penetrative sex or lower back pain. Patients can have one uterine fibroid or many. Fibroids may occasionally cause problems with becoming pregnant, but this is uncommon.
The exact cause of uterine fibroids is unclear. However, fibroids run in families and appear to be partly determined by hormone levels. Risk factors include obesity and eating red meat. Diagnosis can be performed by pelvic examination or medical imaging.
Treatment is typically not needed if there are no symptoms. NSAIDs, such as ibuprofen, and paracetamol (acetaminophen) may help with pain. According to The Mayo Clinic, NSAIDs may help relieve pain tied to fibroids. These medications, however, do not reduce bleeding caused by fibroids as they are not hormonal medicines. Iron supplements may be needed in those with heavy periods. Medications of the gonadotropin-releasing hormone agonist class may decrease the size of the fibroids but are expensive and associated with side effects. If greater symptoms are present, surgery to remove the fibroid or uterus may help. Uterine artery embolization may also help. Cancerous versions of fibroids are very rare and are known as leiomyosarcomas. They do not appear to develop from benign fibroids.
About 20% to 80% of women develop fibroids by the age of 50. In 2013, it was estimated that 171 million women were affected worldwide. They are typically found during the middle and later reproductive years. After menopause, they usually decrease in size. In the United States, uterine fibroids are a common reason for surgical removal of the uterus.
Signs and symptoms Some women with uterine fibroids do not experience any symptoms. However, abdominal pain, anemia, and heavy or increased bleeding can be signs of fibroids. There may also be pain during sexual intercourse, depending on the location of the fibroid. During pregnancy, fibroids may also be the cause of miscarriage, bleeding, premature labor, or interference with the position of the fetus. A uterine fibroid can cause rectal pressure. The abdomen can grow larger, mimicking the appearance of pregnancy. Some large fibroids can extend out through the cervix and vagina.
While fibroids are common, they are not a typical cause for infertility, accounting for about 3% of reasons why a woman may not be able to have a child.American Society of Reproductive Medicine Patient Booklet: Uterine Fibroids, 2003 The majority of women with uterine fibroids will have normal pregnancy outcomes. In cases of intercurrent uterine fibroids in infertility, a fibroid is typically located in a submucosal position. It is thought that this location may interfere with the function of the lining and the ability of the embryo to implant.
Risk factors Some risk factors associated with the development of uterine fibroids are modifiable. Fibroids are more common in obese women. Fibroids are dependent on estrogen and progesterone to grow and therefore are most relevant during the reproductive years.
Diet Diets high in fruits and vegetables tend to lower the risk of developing fibroids. Fibers, vitamin A, C, and E, phytoestrogens, carotenoids, meat, fish, and dairy products are of unclear effect. Normal dietary levels of vitamin D may reduce the risk of developing fibroids.
Genetics Fifty percent of uterine fibroids demonstrate a genetic abnormality. Often, a translocation is found on some chromosomes. Only a few specific genes or cytogenetic deviations are associated with fibroids. 80–85% of fibroids have a mutation in the mediator complex subunit 12 (MED12) gene.
Fibroids are partly genetic. If a mother has fibroids, the risk in the daughter is about three times higher than average.
A 2016 study of patients globally found black women are more likely than white women to develop uterine fibroids, by an estimated 3–9 times higher rate. While African ancestry remains strongly associated with fibroids, newer data from 2025 suggest substantial fibroid burden among several Asian populations as well and women of African descent developing fibroids at younger ages which may result in higher diagnosis rates. Researchers are cautious about interpreting observed racial differences as purely biological because genetics, environment, reproductive factors, healthcare access, medical practices can all contribute to the prevalence and diagnosis of fibroids, and present data may overlook asymptomatic and undiagnosed fibroids.
Familial leiomyomata
A syndrome (Reed's syndrome) that causes uterine leiomyomata along with cutaneous leiomyomata and renal cell cancer has been reported. This is associated with a mutation in the gene that produces the enzyme fumarate hydratase, located on the long arm of chromosome 1 (1q42.3-43). Inheritance is autosomal dominant.
Pathophysiology
Uterine fibroids are leiomyomas that occur in the uterus. Fibroids grossly appear as round, well-circumscribed (but not encapsulated), solid nodules that are white or tan and show a whorled appearance on histological section. The size varies, from microscopic to lesions of considerable size. Typically, lesions the size of a grapefruit or bigger are felt by the patient herself through the abdominal wall.
Microscopically, tumor cells resemble normal cells (elongated, spindle-shaped, with a cigar-shaped nucleus) and form bundles with different directions (whorled). These cells are uniform in size and shape, with scarce mitoses. There are three benign variants: bizarre (atypical), cellular, and mitotically active.
The appearance of prominent nucleoli with peri-nucleolar halos should alert the pathologist to investigate the possibility of the extremely rare hereditary leiomyomatosis and renal cell cancer (Reed) syndrome.
Location and classification
Growth and location are the main factors that determine whether a fibroid causes symptoms and problems. A small lesion can be symptomatic if located within the uterine cavity, while a large lesion on the outside of the uterus may go unnoticed. Different locations are classified as follows: Intramural fibroids are located within the muscular wall of the uterus and are the most common type. Unless they are large, they may be asymptomatic. Intramural fibroids begin as small nodules in the muscular wall of the uterus. With time, intramural fibroids may expand inwards, causing distortion and elongation of the uterine cavity. Subserosal fibroids are located on the surface of the uterus. They can also grow outward from the surface and remain attached by a small piece of tissue. They are called pedunculated fibroids. Submucosal fibroids are located in the muscle beneath the endometrium of the uterus and distort the uterine cavity; even small lesions in this location may lead to bleeding and infertility. A pedunculated lesion within the cavity is termed an intracavitary fibroid and can be passed through the cervix. Cervical fibroids are located in the wall of the cervix (neck of the uterus). Rarely, fibroids are found in the supporting structures (round ligament, broad ligament, or uterosacral ligament) of the uterus that also contain smooth muscle tissue. Since 2011, the FIGO has published its consensus paper on the classification of fibroids, namely from 0 to 8. This is part of the PALM COEIN classification and is the most frequently used in clinical practise and research.
Type 0: pedunculated submucosal, intracavitary (i.e., inside of the uterus) Type 1: submucosal, <50% intramural Type 2: submucosal, ≥50% intramural Type 3: contacts the endometrium, 100% intramural Type 4: intramural (i.e., completely inside the wall of the uterus) Type 5: subserosal, ≥50% intramural Type 6: subserosal, <50% intramural Type 7: pedunculated subserosal Type 8: other (e.g., cervical, parasitic)
There are also hybrid leiomyomas, such as types 2-5, which are both subserosal and submucosal.
Fibroids may be single or multiple. Most fibroids start in the muscular wall of the uterus. With further growth, some lesions may develop towards the outside of the uterus or towards the internal cavity. Secondary changes that may develop within fibroids are hemorrhage, necrosis, calcification, and cystic changes. They tend to calcify after menopause.
If the uterus contains too many to count, it is referred to as diffuse uterine leiomyomatosis.
Extrauterine fibroids of uterine origin, metastatic fibroids Fibroids of uterine origin located in other parts of the body, sometimes also called parasitic myomas, have been historically extremely rare, but are now diagnosed with increasing frequency. They may be related or identical to metastasizing leiomyoma.
They are, in most cases, still hormone-dependent but may cause life-threatening complications when they appear in distant organs. Some sources suggest that a substantial share of the cases may be late complications of surgeries such as myomectomy or hysterectomy. Particularly, laparoscopic myomectomy using a morcellator has been associated with an increased risk of this complication.
There are several rare conditions in which fibroids metastasize. They still grow benignly, but can be dangerous depending on their location. In leiomyoma with vascular invasion, an ordinary-appearing fibroid invades into a vessel, but there is no risk of recurrence. In intravenous leiomyomatosis, leiomyomata grow in veins with uterine fibroids as their source. Cardiac (heart) involvement can be fatal. In benign metastasizing leiomyoma, leiomyomata grow in more distant sites such as the lungs and lymph nodes. The source is not entirely clear. Pulmonary involvement can be fatal. In disseminated intraperitoneal leiomyomatosis, leiomyomata grow diffusely on the peritoneal and omental surfaces, with uterine fibroids as their source. This can simulate a malignant tumor but behaves benignly.
Pathogenesis
Fibroids are monoclonal tumors and approximately 40–50% show karyotypically detectable chromosomal abnormalities. When multiple fibroids are present, they frequently have unrelated genetic defects. Specific mutations of the MED12 protein have been noted in 70 percent of fibroids.
The exact cause of fibroids is not understood. However, the current working hypothesis is that genetic predispositions, prenatal hormone exposure, and the effects of hormones, growth factors, and xenoestrogens cause fibroid growth. Known risk factors are African descent, obesity, polycystic ovary syndrome, diabetes, hypertension, and never having given birth.