Endometrial thickness is a measurement of how thick the lining inside your uterus is at a given point in time. It’s measured in millimeters using a transvaginal ultrasound, and it tells your doctor whether the lining is within a normal range for your age, menstrual phase, or menopausal status. You might see this number on an ultrasound report after a routine exam, a fertility workup, or an evaluation for unusual bleeding.
The endometrium is the tissue that lines the inner wall of the uterus. It thickens each month in response to hormones, preparing to support a potential pregnancy, then sheds during your period if pregnancy doesn’t occur. The thickness of this lining fluctuates constantly, and what counts as “normal” depends entirely on where you are in your cycle or whether you’ve gone through menopause.
How It’s Measured
Endometrial thickness is measured during a transvaginal ultrasound, where a small probe is inserted into the vagina to get a close view of the uterus. The technician or doctor measures the thickest point of the lining from front to back, capturing both layers of the endometrium together in a single measurement. This is a quick, noninvasive process and is considered the go-to first step for evaluating the uterine lining.
Normal Thickness Before Menopause
If you still have periods, your endometrial thickness changes throughout your cycle. During menstruation, the lining is at its thinnest, typically just 1 to 4 mm. After your period ends, around day 7 or 8, it sits around 5 mm. From there, it builds steadily under the influence of estrogen.
By the time you ovulate (around days 13 to 14), the lining reaches roughly 9 to 13 mm and takes on a distinctive three-layered appearance on ultrasound. After ovulation, progesterone kicks in and the lining continues to thicken, peaking at around 11 to 18 mm during the second half of the cycle. A large multicenter study of premenopausal women with regular 28- to 30-day cycles found an average thickness of 7 mm during the first six days, 9.2 mm around ovulation, and 11.1 mm by day 18.
Because the number shifts so much throughout the month, your doctor will always interpret it in context. A 12 mm reading right before your period is perfectly normal. The same reading right after your period would raise questions.
Normal Thickness After Menopause
After menopause, estrogen levels drop significantly and the endometrium thins out. A measurement of 5 mm or less is generally considered normal for someone who is postmenopausal. About 10 to 17% of postmenopausal women without any symptoms will have a uniformly thickened lining between 5 and 11 mm, often from benign causes like small polyps.
The key thresholds your doctor uses depend on whether you’re experiencing bleeding. If you have postmenopausal bleeding and your lining measures over 5 mm, a biopsy is typically recommended. If you have no bleeding but an ultrasound incidentally shows a lining thicker than 11 mm, a biopsy is also worth considering. Below 5 mm with no bleeding, the likelihood of a serious problem is extremely low.
Why Endometrial Thickness Matters for Fertility
For anyone trying to conceive, naturally or through IVF, endometrial thickness is one of the key factors that influences whether an embryo can successfully implant. Think of it as the cushion the embryo needs to attach to and grow. Too thin, and implantation becomes unlikely.
Research on natural-cycle IVF found that 7 mm is the critical threshold. Below that point, pregnancy rates drop sharply. In one study, only a single pregnancy occurred when the lining was under 7 mm, and it ended in early miscarriage. Implantation success improved significantly as thickness increased, plateauing around 10 mm. Both embryo quality and lining thickness independently predicted whether a pregnancy would take hold, but a thin lining was a particularly strong barrier.
If your lining is consistently thin during fertility treatment, your doctor may adjust your hormone protocol to encourage more growth before transferring an embryo.
What Causes a Thick Endometrium
An abnormally thick endometrium most often results from too much estrogen relative to progesterone, a situation sometimes called “unopposed estrogen.” Normally, progesterone rises after ovulation and counterbalances estrogen’s growth signal. When that counterbalance is missing or insufficient, the lining keeps building.
Several common conditions create this hormonal imbalance:
- Polycystic ovary syndrome (PCOS) and other causes of irregular or absent ovulation, since no ovulation means no progesterone surge
- Obesity, because fat tissue produces estrogen independently of the ovaries
- Perimenopause, when cycles become irregular and ovulation is unpredictable
- Estrogen-only hormone therapy without an accompanying progestogen
- Tamoxifen, a breast cancer medication that stimulates the uterine lining
- Endometrial polyps, which are benign growths that cause focal thickening
Starting periods before age 12 and reaching menopause after age 55 also increase cumulative estrogen exposure over a lifetime, raising the risk of lining overgrowth. Rarely, estrogen-secreting ovarian tumors can drive the process. Endometrial hyperplasia, which is the clinical term for this overgrowth, is diagnosed in 25 to 50% of women found to have these tumors.
Symptoms of Abnormal Thickness
A thickened endometrium doesn’t always cause symptoms. In postmenopausal women, it’s sometimes found incidentally on an ultrasound done for another reason. When symptoms do occur, the most common one is abnormal uterine bleeding. This can look different depending on your stage of life:
- Periods that are heavier or last longer than usual
- Menstrual cycles shorter than 21 days
- Any vaginal bleeding after menopause
Postmenopausal bleeding is associated with a 1 to 10% risk of endometrial cancer depending on age and other risk factors. That range is wide, but it’s high enough that any bleeding after menopause warrants evaluation.
When a Thick Lining Needs Further Testing
An ultrasound measurement alone can’t tell your doctor what’s happening at the cellular level. It can show that the lining is thicker than expected, but it can’t distinguish between a harmless polyp, hyperplasia, or cancer. That’s where a biopsy comes in.
An endometrial biopsy involves taking a small sample of lining tissue, usually through a thin tube inserted through the cervix. It’s done in a doctor’s office without anesthesia in most cases and has a sensitivity of up to 97.5% for detecting endometrial cancer. For postmenopausal women without bleeding, a lining over 11 mm carries roughly a 6.7% cancer risk, which is comparable to the risk in women who do have bleeding with a lining over 5 mm. Using 11 mm as the biopsy threshold in asymptomatic women would catch 87% of hidden cancers while requiring the procedure in only about 0.25% of women.
If a biopsy reveals hyperplasia without atypical cells, the condition is often managed with progesterone therapy to counteract estrogen’s effects. Hyperplasia with atypical cells is more concerning because it progresses to cancer in roughly 30 to 45% of cases. That finding typically leads to a referral for more definitive treatment. In some cases, a hysteroscopy, where a small camera is inserted into the uterus, helps visualize polyps or other structural causes of thickening that a biopsy might miss.
What a Thin Lining Can Mean
On the other end of the spectrum, an unusually thin endometrium can also be significant. Low estrogen levels are the most common cause. This can happen naturally after menopause, but in premenopausal women it may point to hormonal issues or be a side effect of certain medications. Long-term use of hormonal birth control that suppresses the cycle can keep the lining thin, which is expected and typically reverses after stopping. Scarring inside the uterus from previous procedures (known as Asherman syndrome) can also prevent the lining from thickening normally. A persistently thin lining is most relevant in a fertility context, where it may reduce the chances of a successful pregnancy.