Endometrial thickness is measured using transvaginal ultrasound, where a technician captures a long-axis view of the uterus and places calipers across the thickest point of the uterine lining. The measurement represents both layers of the endometrium combined (front and back walls) and is recorded in millimeters. It’s a quick, routine measurement, but getting an accurate number depends on proper technique, timing within your menstrual cycle, and whether medications or other factors are changing the lining.
How the Measurement Is Taken
The standard tool is a transvaginal ultrasound, where a small probe is inserted into the vagina to get a close-up view of the uterus. The sonographer finds a sagittal (lengthwise) view of the uterus, then identifies the endometrial “echo,” which is the bright stripe running through the center. Calipers are placed perpendicular to the long axis of that stripe, at the point where the lining appears thickest. The measurement captures the distance from one echogenic border to the other, essentially stacking the front and back layers of the endometrium together into a single number.
One important rule: if there’s fluid inside the uterine cavity, it should not be included in the measurement. The sonographer measures each layer of endometrium separately in that case, then adds them together. Including the fluid would artificially inflate the number.
A transabdominal ultrasound (the kind done over your belly) can also visualize the endometrium, but it’s less precise. Transvaginal imaging gets the probe closer to the uterus, producing sharper images and more reliable measurements.
Bladder Prep: Full vs. Empty
The preparation depends on which type of ultrasound you’re having. For a transabdominal scan, you’ll need a full bladder. Drinking at least 24 ounces of clear fluid about an hour before your appointment pushes the uterus into a better position and moves bowel loops out of the way. For a transvaginal ultrasound, the opposite is true: you should empty your bladder right before the procedure. A full bladder during a transvaginal scan can actually compress the uterus and distort the image.
Many appointments include both approaches, starting with a transabdominal scan on a full bladder and then switching to transvaginal after you use the restroom.
Normal Thickness Through the Menstrual Cycle
Endometrial thickness changes dramatically over the course of a menstrual cycle, which is why the timing of your ultrasound matters. During menstruation, the lining appears as a thin bright line measuring just 1 to 4 mm. Right after your period ends, around day 7 or 8, it typically sits around 5 to 6 mm.
As estrogen rises during the proliferative phase (roughly days 5 through 14), the lining develops a distinctive three-layered “trilaminar” appearance and thickens to about 12 to 13 mm by ovulation, though anywhere from 10 to 16 mm falls within the normal range. In a large multicentric study of premenopausal women with regular 28- to 30-day cycles, the average thickness was 9.2 mm on days 13 to 14.
After ovulation, during the secretory phase, the endometrium becomes more uniformly bright on ultrasound and can reach 16 to 18 mm. By around day 18 the average in that same study was 11.1 mm, tapering down as the cycle approaches menstruation again. So a reading of 15 mm could be perfectly normal in the second half of your cycle but worth investigating if taken right after your period.
What’s Normal After Menopause
Without cycling hormones, the endometrium thins considerably. Canadian gynecology guidelines note that in postmenopausal women without bleeding, an endometrial thickness under 11 mm is associated with roughly a 1% incidence of endometrial cancer. If thickness exceeds 11 mm, or if the ultrasound shows other concerning features like increased blood flow to the lining, uneven texture, or fluid with particles in it, endometrial sampling or referral to a gynecologist is recommended.
These thresholds apply specifically to women without symptoms. For postmenopausal women who are experiencing bleeding, the evaluation is handled differently, and the guidelines for asymptomatic women should not be directly applied. Risk factors like obesity, high blood pressure, late menopause, use of estrogen without progesterone, and genetic cancer syndromes all factor into how aggressively an incidentally discovered thick lining should be investigated.
How Medications Affect the Reading
Certain medications change the endometrium in ways that make standard ultrasound measurements less reliable. Tamoxifen, commonly used in breast cancer treatment, causes a characteristic thickening beneath the surface of the lining. On ultrasound this can look like small cysts just below the endometrial surface, producing a measurement that appears abnormally thick even when the lining itself is normal. Because of this, the correlation between ultrasound thickness and actual pathology is poor in women taking tamoxifen who have no symptoms.
Long-term use of combined oral contraceptive pills also shifts the expected range. Women who have used the pill for 10 or more years tend to have thinner linings. One study found their average endometrial thickness on cycle day 10 was 8.5 mm compared to 9.5 mm in shorter-term users. Even five or more years of use was associated with measurably thinner endometrium. This is particularly relevant for women undergoing fertility treatment, where a thinner lining can affect embryo transfer outcomes.
When Standard Ultrasound Isn’t Enough
A regular transvaginal ultrasound is good at measuring overall thickness but has a notable blind spot: it often cannot tell the difference between a uniformly thickened lining and a focal growth like a polyp. Its sensitivity for detecting endometrial polyps is only about 62%, meaning it misses nearly 4 in 10 cases.
Saline infusion sonohysterography (SIS) solves this problem. During SIS, a small amount of sterile saline is infused into the uterine cavity through a thin catheter while the ultrasound is performed. The fluid separates the two walls of the endometrium, making polyps, fibroids, and other focal growths stand out clearly. For polyp detection, SIS has a sensitivity of about 88% and a specificity of 92%, a substantial improvement. A meta-analysis found that a positive SIS result raises the probability of a polyp from 50% to 89%, while a normal SIS result drops it to just 14%.
SIS is especially useful when a standard ultrasound shows a thick or uneven-looking lining but can’t clarify whether the thickening is diffuse (spread across the whole lining, as with hyperplasia) or focal (a discrete mass). That distinction matters because it determines whether the next step is a biopsy, a hysteroscopy, or simply monitoring.
Factors That Can Distort Results
Beyond medications, several things can make a single measurement misleading. Cycle timing is the most common source of confusion. If your doctor doesn’t know where you are in your cycle, a perfectly normal late-secretory reading of 16 mm could trigger unnecessary concern. When endometrial thickness is being monitored for fertility or screening purposes, the measurement is typically done in the early proliferative phase (around days 5 through 10) to establish a consistent baseline.
Uterine fibroids, especially those that grow into the uterine cavity, can push the endometrial layers apart and create the appearance of a thicker lining. Adenomyosis, where endometrial-like tissue grows into the muscular wall of the uterus, can blur the boundary between endometrium and muscle, making it harder to place calipers accurately. In both cases, the sonographer may note that the measurement is technically difficult or recommend further imaging.
Body habitus can also play a role. In patients where transvaginal imaging is not possible, transabdominal ultrasound provides a less detailed view, and the measurement may be less precise. The transvaginal approach remains the gold standard for endometrial thickness assessment whenever feasible.