The endometrium is the dynamic lining of the uterus that is shed monthly during menstruation if pregnancy does not occur. This tissue grows and thickens under the influence of estrogen to prepare a suitable bed for a fertilized egg. The measurement of this lining, typically obtained through transvaginal ultrasound, is a primary indicator of uterine health. If the measurement exceeds a certain threshold, it can signal underlying conditions.
Defining Endometrial Thickness by Status
The determination of a “thickened” endometrium depends heavily on whether a person is pre-menopausal or post-menopausal, as the normal range changes dramatically throughout life. In pre-menopausal women, the endometrium undergoes expected cyclical changes driven by ovarian hormones. During the menstrual phase, the lining is at its thinnest, typically measuring 2 to 4 millimeters (mm).
Thickness increases to about 5 to 7 mm during the early proliferative phase, reaching up to 11 mm by ovulation. Thickness peaks in the secretory phase, which follows ovulation, measuring between 7 and 16 mm, sometimes up to 18 mm. A measurement exceeding 16 mm in the secretory phase or showing an abnormal contour may prompt further investigation, but there is no single concerning number for pre-menopausal individuals.
For post-menopausal women not taking hormone therapy, the threshold for concern is much lower because the endometrium should be thin and atrophic due to low hormone levels. A measurement of 5 mm or less is considered within the expected range. A measurement exceeding 5 mm, especially with post-menopausal bleeding, often triggers a diagnostic evaluation to rule out pathology. For individuals on hormone replacement therapy (HRT), the lining is expected to be thicker, and a measurement up to 8 mm or even 11 mm may be acceptable depending on the specific regimen.
Common Benign Causes of Thickening
Endometrial thickening is not always indicative of a pre-cancerous or cancerous condition, as several common, non-malignant factors can cause the lining to grow. Localized growths within the uterine cavity, such as endometrial polyps, are a frequent cause of thickening and abnormal bleeding. Polyps are overgrowths of glandular tissue that appear as a focal mass on ultrasound, often resulting from prolonged estrogen stimulation.
Submucosal fibroids, which are benign muscle tumors bulging into the uterine cavity, can also increase measured thickness, though they are structurally distinct from polyps. Certain medications can exert an estrogen-like effect on the uterine lining, leading to generalized thickening. Tamoxifen, used in breast cancer treatment, acts as an estrogen agonist on the endometrium, often causing the lining to measure 9 to 13 mm.
Hormone Replacement Therapy (HRT) can also directly cause endometrial overgrowth, particularly when estrogen is administered without a counter-balancing progestogen. This “unopposed estrogen” stimulates the endometrial tissue to proliferate continuously. Using a combined HRT regimen, which includes both estrogen and progestogen, significantly lowers the risk of thickening and associated hyperplasia.
The Spectrum of Endometrial Hyperplasia
Endometrial hyperplasia is the pathological overgrowth of the glandular components of the endometrium, resulting from chronic, uninterrupted estrogen stimulation. The condition is separated into two main categories based on the appearance of the cells under a microscope. Benign Endometrial Hyperplasia (hyperplasia without atypia) involves structurally crowded glands, but the cells themselves appear normal.
This non-atypical form carries a low risk of progression to cancer, typically less than 5%, and often regresses with hormonal treatment. The more concerning category is Atypical Endometrial Hyperplasia, now classified as Endometrial Intraepithelial Neoplasia (EIN). This diagnosis signifies that the cellular structure is abnormal, indicating a true pre-cancerous lesion.
EIN has a significantly elevated risk of progressing to endometrial cancer, with a cumulative risk of up to 30% if left untreated. Up to 50% of women diagnosed with EIN are found to have a concurrent endometrial carcinoma when a hysterectomy is performed. Because of this malignant potential, EIN requires either definitive surgical management or rigorous medical therapy and surveillance.
Next Steps: Diagnostic Evaluation and Monitoring
When a thickened endometrium is identified, a sequence of diagnostic steps is used to determine the cause and rule out malignancy. Transvaginal Ultrasound (TVUS) is the initial, non-invasive screening tool used to measure the thickness and assess the lining’s general appearance. While effective for measuring overall thickness, TVUS is limited in its ability to differentiate between diffuse, uniform thickening and a focal lesion like a polyp or submucosal fibroid.
If the ultrasound is inconclusive, the next step is often Saline Infusion Sonography (SIS), or sonohysterography. This involves injecting sterile saline into the uterine cavity during a TVUS. The saline distends the cavity, creating a “negative contrast” that clearly outlines focal lesions, such as polyps or fibroids, improving diagnostic accuracy over standard ultrasound.
The definitive diagnosis requires a tissue sample, typically obtained through an Endometrial Biopsy. The Pipelle procedure is a common, low-cost, office-based method that uses a thin suction catheter to collect a sample of the lining. While effective for diagnosing diffuse conditions like hyperplasia, the Pipelle may miss focal lesions and is less reliable than a Dilatation and Curettage (D&C).
The most precise sampling method is Hysteroscopy with directed biopsy, considered the gold standard for evaluating focal lesions. A thin scope is inserted into the uterus, allowing the physician to visually inspect the entire cavity and take a targeted biopsy from any suspicious area. This visual guidance is crucial for accurately diagnosing focal pathology and ensuring a concurrent cancer is not missed.