Normal Endometrial Thickness
Normal endometrial thickness varies dramatically based on menopausal status: in postmenopausal women, the endometrium should measure ≤4 mm, while in premenopausal women, thickness varies cyclically from 3-19 mm depending on menstrual phase, making thickness measurements unreliable for excluding pathology in this population. 1
Postmenopausal Women
The threshold of ≤4 mm effectively excludes endometrial malignancy with a negative predictive value approaching 100%, and no further evaluation is needed if the patient is asymptomatic. 1, 2
Key Thresholds:
- ≤4 mm: Normal, no further workup required in asymptomatic women 1, 2
- ≥5 mm: Warrants endometrial tissue sampling 1, 2
- Mean thickness in asymptomatic postmenopausal women: 2.9 mm (95% CI: 2.6-3.3 mm) 3
The European Society for Medical Oncology uses a slightly more conservative cut-off of 3-4 mm, though the 4 mm threshold from the American College of Radiology is most widely accepted. 4, 1
Important Context:
- Transvaginal ultrasound combined with transabdominal ultrasound is the first-line screening approach for measuring endometrial thickness 1, 2
- Even with thickness <5 mm, abnormal echogenicity and texture may indicate underlying pathology 2
- Body weight and BMI correlate with endometrial thickness in asymptomatic postmenopausal women 5
Premenopausal Women
Endometrial thickness measurement has limited diagnostic value in premenopausal women and should not be used to exclude pathology. 1
Normal Cyclic Variation:
- Follicular phase: Mean 7.8 ± 2.1 mm (range 3-13 mm) 6
- Around ovulation: Mean 10.4 ± 1.9 mm (range 8-13 mm) 6
- Luteal phase: Mean 10.4 ± 2.3 mm (range 8-19 mm) 6
Clinical symptoms, particularly abnormal uterine bleeding, should guide the decision for endometrial sampling rather than thickness measurements alone. 1
Critical Pitfall:
Even with endometrial thickness <5 mm in premenopausal women, endometrial polyps or other pathology may still be present, making thickness an unreliable screening tool in this population. 1
Special Considerations in Early Pregnancy
- <8 mm: Virtually excludes normal intrauterine pregnancy 1
- ≥25 mm: Virtually excludes ectopic pregnancy 1
When to Pursue Tissue Diagnosis
Any postmenopausal woman with endometrial thickness ≥5 mm requires endometrial sampling, regardless of symptoms. 1, 2
- Pipelle or Vabra endometrial sampling devices have sensitivities of 99.6% and 97.1% respectively for detecting endometrial carcinoma 4, 2
- If office-based sampling is inadequate, proceed to hysteroscopy with directed biopsy or fractional curettage 7, 2
- Sonohysterography can distinguish between focal and diffuse pathology when initial ultrasound is inconclusive 7, 2