Normal Endometrial Lining Size
The normal endometrial thickness varies dramatically based on menopausal status: in postmenopausal women, ≤4 mm is normal, while in premenopausal women, thickness ranges from 3-19 mm depending on menstrual cycle phase, making absolute thickness measurements unreliable for determining pathology. 1
Postmenopausal Women
For postmenopausal women, an endometrial thickness of ≤4 mm is considered normal and conveys a negative predictive value for endometrial cancer of nearly 100%. 1 This critical threshold is recommended by the American College of Radiology and represents the standard for reassurance in asymptomatic postmenopausal women 1.
Key Thresholds in Postmenopausal Women:
- ≤4 mm: Normal, no further evaluation needed if asymptomatic 1
- ≥5 mm: Prompts endometrial tissue sampling due to increased cancer risk 1
- Average in asymptomatic women: 1.4 ± 0.7 mm (range 1-5 mm) 2
The European Society for Medical Oncology uses a slightly more conservative cutoff of ≤3 mm, though the 4-5 mm threshold is more widely accepted 1.
Pathological Measurements in Postmenopausal Women:
Research demonstrates that when pathology is present, endometrial thickness increases significantly 2:
- Atrophic endometrium: 2.2 ± 2.0 mm 2
- Endometrial hyperplasia: 17.4 ± 6.7 mm 2
- Endometrial polyps: 16.7 ± 7.7 mm 2
- Endometrial carcinoma: 24.4 ± 11.9 mm 2
Premenopausal Women
In premenopausal women, endometrial thickness is NOT a reliable indicator of pathology because it varies physiologically throughout the menstrual cycle, and there is no validated absolute upper limit cutoff. 1
Normal Cyclic Variations:
The endometrium changes substantially during the menstrual cycle 2, 3:
- Follicular phase (early): Mean 7.8 ± 2.1 mm (range 3-13 mm) 2
- Around ovulation: Mean 10.4 ± 1.9 mm (range 8-13 mm) 2
- Luteal phase: Mean 10.4 ± 2.3 mm (range 8-19 mm) 2
The endometrium increases more during the preovulatory phase (mean 5.5 mm increase) than during the luteal phase (mean 2.6 mm increase) 3.
Clinical Approach in Premenopausal Women:
The American College of Radiology recommends focusing on abnormal echogenicity and texture rather than absolute thickness measurements in premenopausal women. 1 Even with thickness <5 mm, endometrial polyps or other pathology may be present 1.
Clinical symptoms, particularly abnormal uterine bleeding, should drive further evaluation—not thickness measurements alone. 1
Critical Clinical Pitfalls
Never apply postmenopausal thresholds to premenopausal women, as the physiology is fundamentally different. 1 This is a common error that leads to unnecessary interventions in women with normal cyclic variations.
Do not rely solely on endometrial thickness measurement; always consider echogenicity, texture, and clinical presentation together. 1 Transvaginal ultrasound is sensitive for measuring thickness but cannot reliably determine the etiology of thickening 1, 4.