Normal Endometrial Thickness in Menstruating Females
In menstruating women, endometrial thickness varies physiologically throughout the menstrual cycle, ranging from approximately 2-4 mm immediately post-menstruation to 8-16 mm in the late secretory phase, and there is no validated absolute upper limit cutoff for normal thickness in premenopausal women. 1
Cyclical Variation Throughout the Menstrual Cycle
The endometrium undergoes predictable changes in thickness and appearance that correspond to hormonal fluctuations:
Early Follicular Phase (Days 1-7)
- Immediately after menstruation, the endometrium appears as a thin, smooth "pencil line" measuring 2-4 mm 2
- This represents the baseline functional layer after shedding 2
Mid-to-Late Follicular Phase (Days 8-14)
- Endometrial thickness increases steadily at a mean rate of approximately 5.5 mm during the preovulatory phase 2
- The endometrium develops a characteristic trilaminar appearance with an iso- or hypoechoic functional layer 2
- By ovulation, thickness typically reaches 6-10 mm 3, 2
Luteal Phase (Days 15-28)
- The rate of endometrial thickening slows significantly to approximately 2.6 mm during the postovulatory phase 2
- The endometrium becomes thicker and more homogeneously echogenic 2
- Mean thickness in the luteal phase reaches approximately 10 mm (range 8-16 mm) 4
- The endometrium may appear heterogeneous with lower signal intensity on imaging 4
Critical Clinical Distinctions
Why Absolute Thickness Cutoffs Don't Apply
The American College of Radiology explicitly states that endometrial thickness is NOT a reliable indicator of endometrial pathology in premenopausal women, and there is no validated absolute upper limit cutoff. 1 This is fundamentally different from postmenopausal women, where the 4-5 mm threshold has clinical significance 1, 5
What Matters More Than Thickness
In menstruating women, clinical symptoms (particularly abnormal uterine bleeding) should drive further evaluation, not thickness measurements alone 1, 5. The American College of Radiology recommends focusing on:
- Abnormal echogenicity and texture rather than absolute thickness measurements 1
- Vascularity patterns using color/power Doppler to identify structural abnormalities 1
- Structural causes such as polyps, adenomyosis, and leiomyomas, which are the most common sources of abnormal bleeding 1
Common Pitfalls to Avoid
- Never apply postmenopausal thickness thresholds (≤4-5 mm) to premenopausal women—the physiology is fundamentally different 1
- Even with thickness <5 mm, endometrial polyps or other pathology may still be present in premenopausal women 1, 5
- Transvaginal ultrasound should not be used as a screening tool in premenopausal patients due to the wide range of normal endometrial thickness throughout the menstrual cycle 1
When to Pursue Further Evaluation
Rather than relying on thickness measurements, endometrial sampling should be considered based on risk factors for endometrial cancer (age >45 years, obesity, unopposed estrogen exposure) and clinical symptoms, not thickness alone 1