The endometrium is the tissue lining the inside of the uterus. Its primary function is to prepare a hospitable environment for a fertilized egg to implant, a process driven by fluctuating reproductive hormones. If pregnancy does not occur, this lining is shed during menstruation. Measuring endometrial thickness (ET) is a standard diagnostic tool, typically performed using a transvaginal ultrasound. The definition of a “normal” measurement is not a single number but a variable range that depends entirely on a woman’s hormonal status and life stage.
Endometrial Thickness During Reproductive Years
For women in their reproductive years, endometrial thickness directly reflects the phases of the menstrual cycle. This cyclical pattern of growth and shedding is a sign of healthy hormonal responsiveness. Measurements are taken across the thickest part of the lining, which usually reaches a maximum of 16 millimeters (mm) during the cycle.
The cycle begins with the menstrual phase (days one through five), during which the lining sheds. At this time, the endometrium is at its thinnest, generally measuring only 1 to 4 mm. Following menstruation, the proliferative phase (days six through fourteen) begins. Rising estrogen levels stimulate rapid regrowth, causing the lining to thicken from about 5 mm to a pre-ovulatory thickness of up to 11 mm.
The final stage is the secretory or luteal phase, which occurs after ovulation. Progesterone maintains the thickened lining, preparing it for potential implantation. The endometrium typically measures between 7 mm and 16 mm, a range generally considered optimal for embryo implantation. The constant variation in thickness throughout the month is the expected response to hormonal signals.
Defining Normal Thickness After Menopause
Following menopause, the ovaries cease estrogen production, and the endometrium stabilizes at a much thinner, static measurement. The absence of cyclical hormonal stimulation means a thin lining is the expected normal state. For post-menopausal women not on hormone therapy, the endometrium should typically measure less than 5 mm.
Clinical evaluation is often dictated by symptoms, particularly post-menopausal bleeding (PMB). For a woman experiencing PMB, a measurement of 4 mm or less virtually excludes the presence of endometrial cancer. Any measurement exceeding 5 mm in a woman with bleeding warrants immediate further investigation.
For asymptomatic post-menopausal women (those not experiencing bleeding), the acceptable upper limit is less rigid. While the lining is usually very thin, some benign thickening may occur. A higher cutoff, sometimes up to 11 mm, may be considered acceptable in the absence of symptoms, as the risk of cancer is very low below this point. However, a measurement above 11 mm, or any persistent thickening, may still prompt diagnostic follow-up.
Medications and Hormones That Alter Endometrial Thickness
External hormones and certain medications can significantly alter the expected normal thickness, making the interpretation of ultrasound measurements more complex. Hormone Replacement Therapy (HRT) is a common modifier, as it reintroduces estrogen, which stimulates endometrial growth. The acceptable thickness for women on HRT is generally higher than for those not using hormone therapy.
For women on continuous combined HRT (estrogen and progestin), the goal is often a thin, static lining, with an acceptable range suggested up to 8 mm to 11 mm. Sequential HRT, which mimics the natural cycle by providing progestin only part of the month, can cause cyclical thickening and shedding, resulting in a pattern similar to a pre-menopausal cycle, but with different maximum values.
Tamoxifen, a Selective Estrogen Receptor Modulator (SERM) used in breast cancer treatment, also directly impacts the endometrium. While it acts as an anti-estrogen in breast tissue, it behaves as an estrogen agonist in the uterus, causing the lining to thicken significantly. Women on Tamoxifen frequently have mean endometrial thicknesses of 9 mm to 10 mm, even without pathology. For these patients, the presence of bleeding is the main trigger for investigation, as routine screening based on thickness alone is not generally recommended due to the drug’s known proliferative effect.
Hormonal contraceptives, such as progestin-dominant birth control pills or intrauterine devices (IUDs), also modify the lining. Since progestin counteracts the proliferative effects of estrogen, these methods typically suppress and thin the endometrium, often resulting in a very thin, static lining that deviates from the normal cyclical ranges of the reproductive years.
When Endometrial Thickness is Abnormal
When a measurement falls outside the established parameters for a patient’s specific hormonal status, it signals an abnormal state requiring further evaluation. The most common pathology associated with an overly thick lining is endometrial hyperplasia. This condition is caused by prolonged, unopposed exposure to estrogen, which stimulates the tissue to grow excessively.
Endometrial hyperplasia is classified based on the appearance of the cells, and the atypical form carries a significant risk of progressing to endometrial cancer. For example, untreated complex atypical hyperplasia may progress to cancer in up to 30% of cases. Conversely, an abnormally thin lining, or endometrial atrophy, is most relevant in post-menopausal women experiencing bleeding, as it is a common cause of PMB.
An abnormal ultrasound measurement is an indicator, not a definitive diagnosis, and it prompts the next steps in clinical management. If the thickness is concerning, a physician may recommend a Saline Infusion Sonography (SIS), which uses fluid to distend the uterus for a better view of the cavity. The most definitive step is an endometrial biopsy or a procedure called dilation and curettage (D&C) to obtain tissue samples. These samples are then analyzed under a microscope to determine the exact cellular composition and rule out malignancy.