“Endo” on an ultrasound report almost always refers to the endometrium, the lining of the uterus. When your report mentions “endo” followed by a measurement in millimeters, it’s describing the thickness of that lining. Less commonly, “endo” can refer to endometriosis or endometrioma (a type of cyst), or it may appear in the term “endovaginal,” which is simply another name for a transvaginal ultrasound.
Which meaning applies depends on the context of your report. Here’s how to tell the difference and what each one means.
Endometrial Thickness: The Most Common Meaning
The endometrium is the tissue that lines the inside of your uterus. It thickens each month in preparation for a potential pregnancy, then sheds during your period. On an ultrasound report, you’ll often see something like “endo 6mm” or “endometrial stripe 8mm.” This is a straightforward measurement of how thick that lining is at the time of your scan.
What counts as “normal” depends entirely on where you are in your menstrual cycle. During your period, the lining is at its thinnest, typically 2 to 4 mm. During the first half of your cycle (roughly days 6 through 14), the lining rebuilds to about 5 to 7 mm. After ovulation, it continues to thicken, often reaching 10 mm or more as the body prepares for possible implantation of an embryo. If pregnancy doesn’t occur, the cycle resets.
For people who have gone through menopause, the endometrium is expected to be thin since the hormonal cycles that build it up have stopped. If your doctor sees a thicker-than-expected lining on a postmenopausal ultrasound, especially if you’ve had any vaginal bleeding, they will likely recommend a biopsy to collect a small tissue sample. The American College of Obstetricians and Gynecologists now recommends tissue sampling as part of the initial evaluation for most postmenopausal patients with bleeding, rather than relying on a thickness cutoff alone, to improve early detection of uterine cancer.
Endometrioma: Cysts Linked to Endometriosis
If your report mentions an “endometrioma,” that’s a different use of “endo” entirely. An endometrioma is a cyst that forms when endometrial-like tissue grows on or inside an ovary. These cysts fill with old, dark blood, which gives them the nickname “chocolate cysts.” Most are between 2 and 5 centimeters wide.
On ultrasound, endometriomas have a distinctive look. The fluid inside appears as a hazy, uniform gray rather than the clear black of a simple cyst. Sonographers describe this as “ground glass” echogenicity. A typical endometrioma has one to four internal compartments, no solid growths projecting from the wall, and no blood flow detected within the cyst itself. Sometimes blood clots, fibrin strands, or other debris inside the cyst can mimic a solid growth, but the absence of blood flow within that material helps distinguish it from something more concerning.
Finding an endometrioma on ultrasound is a strong indicator that you have endometriosis, a condition where tissue similar to the uterine lining grows in places outside the uterus.
Endometriosis: What Ultrasound Can and Can’t Show
Endometriosis itself can sometimes be detected on ultrasound, but not all forms of it. Ultrasound works well for finding endometriomas on the ovaries and for identifying deeply infiltrating endometriosis, where tissue has grown 5 mm or deeper into organs like the bowel or bladder. It can also reveal large clumps of tissue or scarring. What ultrasound cannot reliably detect is superficial endometriosis, the small, shallow patches of tissue sitting on the surface of organs. Those lesions are typically smaller than 5 mm and don’t show up on imaging.
One technique sonographers use during a transvaginal ultrasound is called the “sliding sign.” The examiner applies gentle pressure with both the internal probe and a hand on your abdomen to see whether the uterus glides freely against the tissue behind it. If the uterus doesn’t slide smoothly, it suggests adhesions or scar tissue may be binding organs together, which is a hallmark of more advanced endometriosis. This simple maneuver is highly accurate, correctly identifying the presence or absence of these adhesions about 93% of the time in research studies.
Endovaginal: A Type of Ultrasound
You may also see “endo” in the term “endovaginal ultrasound.” This is just another name for a transvaginal ultrasound, the kind performed with a slim probe inserted into the vagina rather than rolled across the abdomen. The prefix “endo” here simply means “inside,” referring to the internal approach. Northwestern Medicine and other institutions use the terms interchangeably. If your report header says “endovaginal ultrasound,” it’s describing the procedure itself, not a finding.
How to Read Your Report
Look at the words surrounding “endo” to figure out which meaning applies. A number in millimeters right next to it (like “endo 9mm”) points to an endometrial thickness measurement. A mention of a cyst, mass, or ovarian finding paired with “endo” likely refers to an endometrioma. And if “endo” appears in the title or procedure description, it’s just shorthand for the type of ultrasound that was performed.
If your report includes a measurement that seems unusually high or low, keep in mind that the timing of your scan relative to your menstrual cycle matters enormously. A 12mm endometrium might be perfectly normal on day 22 of your cycle but worth investigating in someone who hasn’t had a period in two years. Your doctor interprets the number in the context of your age, symptoms, menstrual status, and medical history, not as a standalone figure.