What is the normal thickness of the endometrial lining during menstruation?

Normal Endometrial Thickness During Menstruation

The normal endometrial thickness during menstruation is typically at its thinnest, appearing as a smooth, pencil-line endometrium (Type 0) immediately during and after menstruation, as the functional layer has been shed.

Endometrial Thickness Throughout the Menstrual Cycle

The endometrial thickness varies predictably throughout the menstrual cycle:

• During menstruation: The endometrium is at its thinnest as the functional layer is shed
• Follicular phase: The endometrium begins to thicken, developing a trilaminar structure (Type 1) during the mid-follicular phase 1
• Post-ovulation: The endometrium continues to thicken, with more prominent myometrial-endometrial interfaces (Type 2) and eventually becoming homogeneously echogenic (Type 3) 1

According to research, the endometrium increases in thickness more significantly during the preovulatory phase (mean: 5.5 mm) than during the luteal phase (mean: 2.6 mm) 1.

Imaging Considerations

When evaluating endometrial thickness, transvaginal ultrasound (TVUS) should be combined with transabdominal ultrasound for a complete assessment of pelvic structures 2:

• Transabdominal ultrasound provides an anatomic overview
• Transvaginal ultrasound offers greater spatial and contrast resolution
• Color and spectral Doppler should be used to evaluate internal vascularity of pelvic findings

Key Points for Clinicians

• In premenopausal women, normal endometrial thickness varies with the phase of the menstrual cycle 2
• During menstruation, the endometrium appears as a thin, pencil-line structure (Type 0) 1
• The endometrial thickness measurement should be a double-layer measurement (anterior and posterior walls together)

Clinical Implications

Understanding normal endometrial thickness during menstruation is important for:

1. Establishing baseline measurements for individual patients
2. Distinguishing normal from pathological findings
3. Timing certain procedures (e.g., endometrial biopsy) appropriately

Common Pitfalls to Avoid

• Measurement timing: Failure to account for the day of the menstrual cycle when interpreting endometrial thickness
• Technique errors: Not obtaining proper double-layer measurements
• Misinterpretation: Confusing intracavitary fluid with abnormal endometrial thickening
• Incomplete evaluation: Not using both transabdominal and transvaginal approaches when indicated

For postmenopausal women (for reference), an endometrial thickness ≤4 mm conveys a negative predictive value for cancer of nearly 100% 2, 3. However, this threshold does not apply to women during active menstruation, where the endometrium is expected to be thin as part of the normal physiological process.