Leiomyoma of the Uterus: Understanding Your Pathology Report

Section Editor: Kianoosh Keyhanian MD FRCPC
September 2, 2026

A leiomyoma is a noncancerous (benign) tumor that grows in the muscular wall of the uterus. It is made of smooth muscle cells, the same cells that make up the uterine wall, called the myometrium. In the uterus, leiomyoma and fibroid mean the same thing. Pathologists tend to write leiomyoma, and everyone else tends to say fibroid.

Leiomyomas are the most common tumor of the female reproductive tract. By the age of 50, roughly 70 out of every 100 white women and around 80 out of every 100 Black women have at least one. Many never know it. Most people who have them have more than one.

A leiomyoma is not cancer, and it does not spread to other parts of the body. This article will help you understand what this diagnosis means on your pathology report, what each term means, and why it matters for your care.

What causes a leiomyoma?

Each leiomyoma begins when a single smooth muscle cell in the wall of the uterus acquires a genetic change and starts dividing more than it should. All the cells in one fibroid descend from that original cell, which is why a person can have several fibroids that behave quite differently from one another.

The genetic changes involved are usually acquired during life rather than inherited. A gene called MED12 is altered in most fibroids, and changes affecting a gene called HMGA2 account for many of the rest. Neither is tested for in routine practice, and neither will appear on your pathology report.

Growth depends on estrogen and progesterone, which is why fibroids appear during the reproductive years, may enlarge during pregnancy, and usually shrink after menopause. Several factors make them more likely.

Age — Fibroids become more common through the thirties and forties.
Race — Black women develop fibroids earlier, more often, and in greater numbers, and the tumors tend to be larger. The reasons are not fully understood.
Family history — Having a mother or sister with fibroids raises the likelihood.
Early first period — Starting periods at a younger age means longer exposure to the hormones that drive growth.
Excess body weight — Fat tissue converts other hormones into estrogen.

Having given birth several times is associated with a lower likelihood of developing fibroids.

What are the symptoms?

Most fibroids cause no symptoms at all, and many are found by chance during an examination, an imaging test, or surgery performed for another reason. When symptoms do occur, they depend more on where the fibroid sits than on its size.

Heavy or prolonged periods — The most common symptom, and the most common reason for treatment. Heavy bleeding over time can lead to iron deficiency and anemia.
Pelvic pressure or fullness — Caused by the bulk of a larger fibroid or of several together.
Pelvic or abdominal pain — Sometimes ongoing, sometimes sudden if a fibroid outgrows its blood supply.
Pressure on nearby organs — A fibroid pressing on the bladder can mean needing to urinate more often. One pressing on the bowel can cause constipation or a feeling of fullness.
Difficulty becoming pregnant — Fibroids that distort the cavity of the uterus can interfere with pregnancy. Most fibroids do not affect fertility.

Where in the uterus can a leiomyoma be found?

Your report and your imaging results will describe where each fibroid sits in the wall of the uterus. This is the single most useful piece of information for explaining your symptoms, because location matters more than size.

Submucosal — Just beneath the lining of the uterus, called the endometrium, and often bulging into the cavity. These cause the heaviest bleeding and are the ones most likely to affect fertility, even when small.
Intramural — Within the muscular wall itself. The most common location. Larger ones can cause both bleeding and pressure.
Subserosal — Just beneath the smooth outer covering of the uterus, called the serosa. These tend to press on neighboring organs rather than cause bleeding.
Pedunculated — Attached by a narrow stalk, either inside the cavity or on the outer surface. A stalk can twist, causing sudden, severe pain.
Cervical — Arising in the wall of the cervix rather than the body of the uterus. Uncommon.

How is the diagnosis made?

Fibroids are usually identified on an ultrasound, and MRI is used when more detail is needed about size, number, and position. A saline infusion sonohysterogram, in which fluid is placed in the uterus during a scan, shows submucosal fibroids particularly well.

A pathologist makes a definite diagnosis by examining the tissue under a microscope. The tissue reaches the laboratory after a myomectomy, in which the fibroids alone are removed, or after a hysterectomy. In many cases, the uterus was removed for another reason, and the fibroid was an incidental finding.

Imaging cannot tell the different types of fibroid apart, and it cannot reliably separate a fibroid from a rare cancer of the same tissue. That distinction is made under the microscope.

What does a leiomyoma look like under the microscope?

A leiomyoma is a benign smooth muscle tumor of the uterus, well defined and separate from the surrounding wall. To the naked eye, it is firm and white, with a whorled cut surface, and it bulges when cut. Under the microscope, the pathologist looks for the following features.

Spindle cells in bundles — The tumor is made of long, thin cells called spindle cells, arranged in interlacing bundles called fascicles.
Uniform nuclei — The cells look alike, with blunt-ended nuclei often described as cigar-shaped. Size and shape show little variation.
Few dividing cells — A cell caught in the act of dividing is called a mitotic figure. These are infrequent in an ordinary fibroid.
No tumor cell necrosis — There is no tumor cell death of the type associated with cancer.
Thick-walled blood vessels — A characteristic feature of fibroids.
Cystic spaces — Open, fluid-filled spaces described as cysts are common, particularly in larger tumors.

What do degenerative changes and infarct-like necrosis mean?

As fibroids grow and age, parts of the tumor break down. Pathologists call this degenerative change, and it is very common, particularly in larger fibroids that have been present for a long time. Your report may describe the tissue as hyalinized, cystic, myxoid, or calcified, all of which are forms of this breakdown.

Some fibroids grow so large that blood cannot reach the entire tumor. Cells that lose their blood supply die, a process called necrosis. When the pattern resembles an infarct, meaning tissue killed by loss of blood supply, it is called infarct-like necrosis. This is expected after treatment with hormone medication or after uterine artery embolization, a procedure that deliberately blocks blood flow to shrink a fibroid. A particular form called red degeneration can occur during pregnancy and causes sudden pain.

The word necrosis also appears in the criteria used to diagnose cancer, so seeing it in a benign report is understandably confusing. The two are not the same. Infarct-like necrosis shows a zone of healing tissue between the dead and living areas. Tumor cell necrosis, the kind that counts toward a diagnosis of cancer, has an abrupt border with no healing zone. Distinguishing between them is a routine part of the pathologist’s assessment.

What do the different types of leiomyoma mean?

Most fibroids are reported simply as leiomyoma. Some have a feature that earns a more specific name. The names below all describe benign tumors, and all are treated the same way.

Cellular leiomyoma — The cells are packed more tightly together, with less collagen between them. A cellular leiomyoma is benign.
Mitotically active leiomyoma — An increased number of dividing cells, with normal-looking cells and no tumor cell necrosis. A mitotically active leiomyoma is benign, and the increased count usually reflects the person’s hormonal state.
Leiomyoma with bizarre nuclei — Also called atypical or symplastic leiomyoma. Some of the cells are larger and darker than normal, but dividing cells are few, and there is no tumor cell necrosis. These behave benignly in almost all cases.
Myxoid or epithelioid leiomyoma — Uncommon forms in which the tumor contains gelatinous material, or is made of rounded rather than spindle-shaped cells. These are assessed with more caution, because the criteria separating benign from malignant differ for them.

One point about bizarre nuclei is worth knowing. A proportion of these tumors result from loss of a gene called fumarate hydratase. When a pathologist sees that pattern, the report may recommend testing. In some families the same gene change is inherited, and it is linked to a condition called hereditary leiomyomatosis and renal cell cancer. That condition carries a risk of kidney cancer that benefits from early detection. Most fibroids have nothing to do with it, but if your report raises fumarate hydratase or recommends genetic assessment, follow that up rather than setting it aside.

Can a leiomyoma turn into cancer?

The short answer is no. A uterine leiomyosarcoma is a cancer made of the same kind of smooth muscle cells. Evidence indicates these cancers arise on their own rather than developing from an existing fibroid.

The numbers are reassuring when you put them side by side. Around 70 in every 100 women develop fibroids at some point, while leiomyosarcoma affects roughly 6 people in every million each year. Pathologists separate the two using three features together: how many cells are dividing, whether the cells look abnormal, and whether tumor cell necrosis is present. A diagnosis of cancer generally requires a combination of these rather than any one alone.

Occasionally a tumor has a mixture of features that fits neither category cleanly. That is reported as a smooth muscle tumor of uncertain malignant potential, or STUMP. It is not cancer, but its behavior is less predictable, and it is followed more closely.

How are leiomyomas treated?

Treat fibroids only when they cause problems. Many need nothing at all, and most shrink after menopause. The right approach depends on your symptoms, the size and location of the fibroids, and whether you hope to become pregnant.

Watchful waiting — Reasonable for fibroids causing no symptoms, whatever their size.
Medication for bleeding — Options include tranexamic acid, anti-inflammatory medication, hormonal contraceptives, and the hormonal IUD, which often reduces bleeding substantially.
Medication to shrink fibroids — Medications that lower ovarian hormone levels can reduce fibroid size, often before surgery.
Uterine artery embolization — A procedure that blocks the blood supply to fibroids so they shrink, without removing them.
Myomectomy — Surgical removal of the fibroids with the uterus left in place. The usual choice for those who wish to preserve fertility. New fibroids can develop afterward.
Hysterectomy — Removal of the uterus. The only treatment after which fibroids cannot return.

What other findings may be described in the report?

Your report will describe the fibroids and the rest of the uterus. Findings commonly listed include:

Size and number — Each fibroid is measured, and the report gives the size of the largest.
Location — Submucosal, intramural, or subserosal, as described above.
The lining of the uterus — If a hysterectomy was performed, the endometrium is examined and reported separately. An endometrial polyp or endometrial hyperplasia may be reported alongside the fibroids.
Adenomyosis — Endometrial-type tissue within the muscular wall. A common accompanying finding that can cause similar symptoms and is often diagnosed only after surgery.
Endometriosis — Endometriosis may be found on the outer surface of the uterus or elsewhere in the pelvis.

Questions to ask your doctor

How many fibroids were found, and how large was the largest?
Where in the uterus were they?
Which fibroid was most likely responsible for my symptoms?
Does my report use a specific type, such as cellular, mitotically active, or bizarre nuclei?
My report mentions necrosis. Is that the benign kind?
Does my report mention fumarate hydratase or recommend genetic testing?
Did you consider a leiomyosarcoma, and what ruled it out?
Were the fibroids removed completely, or was the uterus left in place?
Could new fibroids develop, and how likely is that?
What does this mean for my fertility or a future pregnancy?
Was anything found in the lining of my uterus?
Do I have adenomyosis as well?
What treatment options do I have if symptoms return?