Uterine fibroids are most often diagnosed through a combination of a pelvic exam and an ultrasound. In many cases, fibroids are discovered incidentally during a routine gynecological visit when a doctor feels an enlarged or irregularly shaped uterus. From there, imaging confirms the finding, determines the size and number of fibroids, and pinpoints exactly where they’re growing.
What Happens During the Pelvic Exam
A fibroid diagnosis often starts with a standard pelvic exam. Your doctor presses on your abdomen and feels for changes in the shape and size of your uterus. Fibroids typically present as a firm, irregular mass. A uterus with large fibroids can feel noticeably enlarged, sometimes comparable in size to a pregnancy of several weeks. Small fibroids, however, are easy to miss on a physical exam alone, which is why imaging almost always follows when fibroids are suspected or when you report symptoms like heavy periods, pelvic pressure, or pain.
Ultrasound: The Standard First Step
Transvaginal ultrasound is the go-to imaging tool for confirming fibroids. A small probe is inserted into the vagina, producing detailed images of the uterus. On ultrasound, fibroids typically appear as well-defined, solid, round masses that are darker than the surrounding muscle tissue. They often cast acoustic shadows, similar to how a rock blocks light, which helps distinguish them from other structures.
Not all fibroids look the same on ultrasound. Those with more calcium deposits or fibrous tissue can appear brighter on the screen. Fibroids that have outgrown their blood supply and started to break down internally may show dark, fluid-filled areas. Occasionally, a small fibroid blends in with the uterine wall so closely that the only visible clue is a subtle bulge in the uterine contour. This variability is one reason your doctor may order additional imaging if the initial ultrasound doesn’t tell the full story.
Saline Sonography for a Closer Look
When a fibroid grows into or near the uterine cavity (the space where a pregnancy would develop), standard ultrasound sometimes can’t show exactly how much of the fibroid is protruding inward. Saline infusion sonography, sometimes called a sonohysterogram, solves this problem. A thin catheter is placed through the cervix, and sterile saline is gently pushed into the uterine cavity. The fluid separates the walls of the uterine lining, creating a clear contrast that reveals the shape and extent of any growths along the inner surface.
This procedure is done in an office setting and is generally well tolerated. It’s less invasive and less expensive than surgical alternatives like hysteroscopy, where a camera is inserted into the uterus. Saline sonography is particularly useful if you’re experiencing abnormal bleeding or if your doctor needs to assess whether a fibroid could affect fertility or complicate a future pregnancy.
When MRI Is Needed
MRI is more sensitive than ultrasound at identifying fibroids and is the preferred method for accurately characterizing pelvic masses. It’s not used for every patient, but your doctor may recommend it in several specific situations: when ultrasound results are unclear, when the uterus contains many fibroids that are hard to map individually, when a pelvic mass could be something other than a fibroid, or when you’re being evaluated for a procedure like uterine artery embolization (a treatment that shrinks fibroids by cutting off their blood supply).
MRI excels at showing the internal structure of each fibroid. It can reveal whether a fibroid has a healthy blood supply or has started to degenerate. A type of degeneration called red or carneous degeneration, caused by hemorrhagic infarction when veins draining the fibroid become blocked, produces distinctive signals on MRI that help doctors identify it without surgery. When a contrast dye is used during the scan, doctors can directly assess how much blood flow reaches a fibroid, which helps predict whether embolization is likely to work well.
MRI also clearly shows the uterus’s layered anatomy, making it easier to determine exactly where a fibroid sits relative to the inner lining and outer surface. This level of detail matters when planning treatment.
How Fibroids Are Classified by Location
Once imaging confirms fibroids, doctors classify them using a standardized numbering system developed by FIGO (the International Federation of Gynecology and Obstetrics). This system assigns each fibroid a type from 0 to 8 based on where it grows in relation to the uterine lining, the muscle wall, and the outer surface of the uterus. The classification directly influences which symptoms you experience and which treatments are appropriate.
Type 0 fibroids sit entirely inside the uterine cavity, attached by a stalk with no involvement in the muscle wall. Types 1 and 2 are submucosal fibroids that push into the cavity but are partially embedded in the muscle, with Type 1 having less than half its diameter in the wall and Type 2 having half or more. Type 3 fibroids touch the inner lining but don’t distort it. Type 4 fibroids are entirely within the muscle wall, surrounded on all sides by uterine tissue. Types 5, 6, and 7 mirror the submucosal pattern but on the outer surface: Type 5 is mostly embedded in the wall, Type 6 is mostly outside it, and Type 7 hangs off the outer surface on a stalk. Type 8 covers fibroids in unusual locations, such as the cervix or those that have detached from the uterus entirely.
Some fibroids span the full thickness of the uterine wall, touching both the inner lining and the outer surface. These are classified as hybrids, labeled with two numbers like “2-5,” indicating both a submucosal and subserosal component. The 2018 revision of this system also standardized reporting of fibroid location (front, back, or sides of the uterus), position within the upper or lower half, and total uterine volume.
Blood Tests That Support the Diagnosis
Blood tests don’t diagnose fibroids directly, but they play an important role in the workup. If you’re experiencing heavy or irregular periods, your doctor will likely order a complete blood count to check for anemia from ongoing blood loss. Low hemoglobin or iron stores can confirm that your bleeding is significant enough to affect your health, which helps guide treatment decisions. Additional blood tests may be ordered to rule out other causes of abnormal bleeding, including thyroid disorders and bleeding disorders, since these conditions can mimic or worsen fibroid symptoms.
Distinguishing Fibroids From Adenomyosis
One of the trickier aspects of fibroid diagnosis is telling them apart from adenomyosis, a condition where tissue similar to the uterine lining grows into the muscle wall. Both cause heavy bleeding and an enlarged uterus, and they frequently coexist. On ultrasound, fibroids appear as distinct masses with clear borders, surrounded by normal muscle tissue on at least 75% of their perimeter, often with calcifications and shadowing. Adenomyosis looks different: the uterus takes on a globular shape, the muscle wall appears uneven or asymmetric, and small cysts may appear just beneath the lining.
Distinguishing the two on ultrasound depends heavily on the skill of the person performing the scan, and accuracy varies widely. MRI generally does a better job when the ultrasound is inconclusive. Newer techniques like shear wave elastography, which measures tissue stiffness, have been studied as a potential additional tool, but research so far shows it cannot reliably differentiate fibroids from adenomyosis. Getting the distinction right matters because the two conditions respond differently to treatment.