What is the normal endometrial thickness (ET) in the reproductive age group, cycle-wise?

Normal Endometrial Thickness in Reproductive Age Women by Menstrual Cycle Phase

In reproductive age women, normal endometrial thickness varies significantly throughout the menstrual cycle, ranging from approximately 2-4 mm during menstruation to 8-14 mm in the late secretory/luteal phase, with no validated absolute upper limit cutoff for premenopausal women. 1

Cycle-Wise Endometrial Thickness Patterns

Early Follicular Phase (Days 1-7)

• Endometrial thickness measures 2-5 mm immediately following menstruation 1
• The endometrium appears as a thin, echogenic line on ultrasound during this phase 1

Late Follicular/Proliferative Phase (Days 8-14)

• Endometrial thickness increases progressively to approximately 6-11 mm by ovulation 1, 2
• A rapid growth pattern occurs with a daily growth rate of approximately 0.5 mm per day during gonadotropin-stimulated cycles 3
• In natural cycles, the endometrium typically reaches 9-12 mm around the time of ovulation 3

Luteal/Secretory Phase (Days 15-28)

• Endometrial thickness reaches its maximum at 8-14 mm (median 10.4 mm, range 6.0-17.3 mm) 2
• Growth rate slows to approximately 0.1 mm per day in the early-to-mid luteal phase 3
• The endometrium maintains thickness of approximately 10-13 mm throughout most of the luteal phase 3, 2

Clinical Significance and Interpretation

Normal Variations

• There is no validated absolute upper limit for endometrial thickness in premenopausal women, making interpretation highly dependent on cycle timing 1
• Endometrial thickness of 6.6 mm may be entirely normal depending on menstrual cycle phase in reproductive age women 1
• The presence of endometrial folds is common in the luteal phase, occurring in 46% of women (95% CI, 27-65%) 2

Important Caveats for Assessment

• Endometrial thickness does not correlate with serum estradiol or progesterone levels in stimulated cycles 3
• Ultrasound is sensitive for measuring endometrial thickness but cannot reliably determine the etiology of endometrial thickening 4
• Abnormal echogenicity and texture of the endometrium correlate with underlying pathology regardless of thickness 1, 4

Diagnostic Pitfalls to Avoid

• Do not perform saline contrast sonohysterography in the luteal phase, as endometrial folds are common and may lead to overdiagnosis of focal pathology such as polyps 2
• If sonohysterography results are equivocal in the luteal phase, repeat the procedure in the follicular phase 2
• Timing of ultrasound assessment is critical—always correlate endometrial thickness with the patient's menstrual cycle day 1, 2
• In fertility evaluations, conception is unlikely with endometrial thickness below 13 mm on day +11 post-ovulation trigger in stimulated cycles 3

Contrast with Postmenopausal Women

• In postmenopausal women, endometrial thickness should be ≤4 mm, with investigation warranted at ≥5 mm 5, 4
• The negative predictive value for cancer approaches 100% when endometrium measures ≤4 mm in postmenopausal women 1, 4
• This threshold does not apply to reproductive age women, where significantly greater thickness is physiologically normal 1