Normal endometrial thickness in postmenopausal women is generally 4 mm or less. The endometrium, the lining of the uterus, thins naturally after menopause because estrogen levels drop and the monthly cycle of building and shedding that lining stops. This measurement matters because it’s one of the key ways doctors screen for endometrial cancer, and the threshold that triggers concern depends on whether you’re experiencing bleeding.
The 4 mm Cutoff and Why It Matters
The number you’ll hear most often is 4 mm, sometimes 5 mm. ACOG uses 4 mm or less as the reassuring threshold for postmenopausal women who have vaginal bleeding. At that thickness, the negative predictive value for endometrial cancer is greater than 99%, meaning cancer is extremely unlikely. The sensitivity of this cutoff is about 96%, which means it correctly flags the vast majority of cancers when the lining is thicker than 4 mm.
In practice, 4 mm and 5 mm cutoffs perform almost identically. Studies comparing cutoffs of 3, 4, and 5 mm found nearly the same sensitivity (96.2%, 95.7%, and 96.2%) and negative predictive values (99.7%, 99.4%, and 99.2%). The 4 mm line has become the standard because it offers the best balance between catching real problems and avoiding unnecessary procedures.
With Bleeding vs. Without Bleeding
Whether you’re experiencing postmenopausal bleeding changes how your doctor interprets the measurement. For women with bleeding, a lining of 4 mm or less is generally reassuring enough that no tissue sampling is needed as a first step. Above 4 mm in the presence of bleeding, further evaluation is typically recommended.
For women without any bleeding whose thickened endometrium is found incidentally on an ultrasound done for another reason, the rules are different. ACOG states that a measurement above 4 mm discovered without bleeding does not routinely need further workup, though your individual risk factors matter. Research suggests that in asymptomatic women, a significant cancer risk only emerges at 11 mm or above. At that thickness, the cancer risk is about 6.7%, roughly three times the risk seen in asymptomatic women with measurements between 5 and 10 mm. Below 11 mm without bleeding, the 4 mm cutoff used for symptomatic patients has a poor positive predictive value and shouldn’t be applied the same way.
One important exception: rare types of endometrial cancer can appear even when the lining is very thin, under 3 mm. If you have persistent or recurring postmenopausal bleeding, tissue sampling is warranted regardless of what the ultrasound shows.
How the Measurement Is Taken
Endometrial thickness is measured with a transvaginal ultrasound, where a small probe is inserted into the vagina to get a close-up view of the uterus. The measurement captures both layers of the endometrium together (front wall and back wall) in a single number. This “double-layer” measurement is what the 4 mm and 5 mm cutoffs refer to.
The test is noninvasive, takes only a few minutes, and doesn’t require anesthesia. It’s considered a reasonable first step for evaluating an initial episode of postmenopausal bleeding, and your doctor doesn’t need to perform both an ultrasound and a biopsy at the same time. One or the other is sufficient as a starting point.
How Hormone Therapy Affects Thickness
If you’re taking hormone replacement therapy, your endometrial thickness may be higher than the standard postmenopausal range, and the type of hormone regimen matters. Women on sequential hormone therapy (where a progestogen is added for part of the cycle) have an average endometrial thickness of about 8 mm, compared to 5 mm in women not using hormones. That’s a significant difference that can trigger unnecessary worry if you don’t know it’s expected.
Sequential hormone users also show more variation in their measurements over the course of a cycle. More than half of women on sequential hormones had thickness swings greater than 3 mm, compared to about 10% of women on no hormones. The lining tends to be thickest between days 13 and 23 of the hormone cycle, so if your doctor is monitoring your endometrium, timing the ultrasound early or late in the cycle gives a more accurate baseline reading.
Women on continuous combined hormone therapy (where estrogen and progestogen are taken together every day) tend to have less fluctuation, though their measurements may still run slightly higher than women on no hormones at all.
Tamoxifen and Thickened Lining
Tamoxifen, commonly used in breast cancer treatment, is a well-known cause of endometrial thickening that can confuse ultrasound results. Nearly half of women taking tamoxifen show an endometrial thickness of 8 mm or more on ultrasound, even when they have no symptoms. This happens because tamoxifen has estrogen-like effects on the uterine lining.
Among those with thickness at 8 mm or above, many turn out to have a normal endometrium on biopsy. The most common actual finding is endometrial polyps, which appear in about a third of biopsies in this group. Endometrial cancer is found in a small minority. Because of this high rate of false alarms, the standard 4 to 5 mm cutoff isn’t directly applicable to women on tamoxifen. If you’re taking this medication, your doctor will interpret your ultrasound differently and may use 8 mm as a more appropriate reference point.
What Can Cause a Thicker Lining
Beyond hormone therapy and tamoxifen, several other factors can cause the endometrium to measure above 4 mm after menopause:
- Endometrial polyps: These are small, usually benign growths on the uterine lining. They’re one of the most common causes of a thickened endometrium and postmenopausal bleeding.
- Endometrial hyperplasia: An overgrowth of the lining, sometimes caused by excess estrogen without enough progesterone to balance it. Some types are considered precancerous.
- Obesity: Fat tissue produces estrogen, so women with a higher body weight may have a thicker endometrium even after menopause.
- Endometrial cancer: A thickened lining is one of the earliest signs, which is why thickness measurement is used as a screening tool.
A single ultrasound measurement above 4 mm doesn’t mean something is wrong. It means additional information is needed, whether that’s a repeat ultrasound, a biopsy, or simply a conversation about your risk factors and symptoms. The number is a screening tool, not a diagnosis.