Normal Endometrial Thickness
In postmenopausal women, an endometrial thickness of ≤4 mm is considered normal and effectively excludes endometrial malignancy, while in premenopausal women, endometrial thickness varies throughout the menstrual cycle and has limited diagnostic value. 1
Postmenopausal Women
The American College of Radiology establishes ≤4 mm as the normal threshold for postmenopausal women, providing a negative predictive value for endometrial cancer of nearly 100%. 1, 2
Key Thresholds:
- ≤4 mm: Normal; no further evaluation needed if asymptomatic 1, 2
- ≥5 mm: Abnormal; endometrial tissue sampling generally warranted 1, 2
- Mean thickness in asymptomatic postmenopausal women: 2.9 mm (95% CI, 2.6-3.3 mm) 3
Important Caveat:
The European Society for Medical Oncology uses a slightly more conservative cut-off of 3-4 mm, which may be considered in high-risk patients. 1, 2 However, the 4 mm threshold from the American College of Radiology represents the most widely accepted standard in clinical practice. 1
Premenopausal Women
Endometrial thickness measurement has limited diagnostic value in premenopausal women and should not be used to exclude pathology. 1
Normal Thickness by Cycle Phase:
- Follicular phase: 7.8 ± 2.1 mm (range 3-13 mm) 4
- Around ovulation: 10.4 ± 1.9 mm (range 8-13 mm) 4
- Luteal phase: 10.4 ± 2.3 mm (range 8-19 mm) 4
Critical Pitfall:
Even if endometrial thickness is <5 mm in premenopausal women, endometrial polyps or other pathology may still be present. 1 Clinical symptoms, particularly abnormal uterine bleeding, should guide the decision for endometrial sampling rather than thickness measurements alone. 1
Diagnostic Approach
Transvaginal ultrasound (TVUS) is the first-line screening test for evaluating endometrial thickness, particularly in postmenopausal women with bleeding. 1
Recommended Imaging Protocol:
- Combine TVUS with transabdominal ultrasound for complete pelvic assessment 1, 2
- TVUS is sensitive for measuring thickness but cannot reliably determine the etiology of endometrial thickening 2
- Abnormal echogenicity and texture correlate with significant pathology even when thickness appears normal 2
Special Clinical Contexts
Early Pregnancy Evaluation:
- <8 mm: Virtually excludes normal intrauterine pregnancy 1
- ≥25 mm: Virtually excludes ectopic pregnancy 1
When to Sample Despite "Normal" Thickness:
In postmenopausal women with persistent bleeding, endometrial sampling may be warranted even with thickness ≤4 mm if repeat imaging confirms the measurement and clinical suspicion remains high. 2 The negative predictive value remains nearly 100% with confirmed measurements <4 mm on repeat imaging. 2