Expected Endometrial Thickness by Menstrual Cycle Phase
The endometrium progressively thickens from approximately 4.5 mm in the early follicular phase to 10–12 mm at ovulation, then plateaus throughout the luteal phase at approximately 10–12 mm.
Follicular Phase (Early to Mid)
- Early follicular phase (menstruation to day -7): Endometrial thickness ranges from 4.5 ± 0.8 mm 1.
- During this period, ultrasound shows only a single thin echogenic line, reflecting the thin, post-menstrual endometrium 2.
- The endometrium appears as a smooth, pencil-line structure (Type 0) during and immediately after menstruation 3.
Late Follicular Phase (Pre-ovulatory)
- Mid to late follicular phase (day -6 to day -1): The endometrium transitions to a trilaminar pattern with three distinct echogenic lines and increasing hypoechogenic texture between them 2.
- Endometrial thickness increases significantly during this phase, with a mean growth of approximately 5.5 mm during the preovulatory period 3.
- By day -1 (just before ovulation), the endometrium demonstrates maximum hypoechogenic texture between thickened lines, corresponding to rising estrogen levels 2.
Ovulatory Phase
- At ovulation (day 0): Mean endometrial thickness reaches 12 mm (measuring both layers combined) 2.
- Another study reports the dominant follicle diameter peaks at 18.5 ± 2.0 mm at ovulation, with endometrial thickness reaching its maximum 1.
- The trilaminar pattern persists with maximum hypoechogenic texture between the lines 2.
Luteal Phase
- Early to mid-luteal phase (day +1 to +6): The three echogenic lines begin to blur, and the previously hypoechogenic layers gradually display more echogenic texture 2.
- Late luteal phase (day +7 to menstruation): The endometrium becomes homogeneously hyperechogenic (Type 3), with indistinguishable lines due to general hyperechogenic texture 2, 3.
- Endometrial thickness reaches 10.2 ± 1.1 mm and remains relatively stable throughout the luteal phase, with a mean growth of only 2.6 mm during this phase 1, 3.
- Both endometrial volume and thickness plateau around ovulation and remain stable throughout the luteal phase 4.
Clinical Correlation
- The progressive increase in endometrial thickness from follicular to ovulatory phase is highly correlated with rising estrogen levels (mean: 180 ± 20 pg/mL during follicular phase) 1.
- The luteal phase plateau corresponds to elevated progesterone levels (mean: 15 ± 3 ng/mL) 1.
- Endometrial thickness and volume changes are highly correlated (R² = 0.767) throughout the cycle 4.
Important Caveats
- No validated upper limit exists for premenopausal endometrial thickness because measurements vary significantly with cycle phase; pathology can be present even when thickness is < 5 mm 5.
- For a given endometrial thickness, the stage of endometrial development can vary widely, meaning thickness alone cannot accurately predict histological dating 6.
- Clinical symptoms (abnormal uterine bleeding, pelvic pain) should guide decisions for tissue sampling in premenopausal patients rather than thickness measurements alone 5.
- Abnormal endometrial echogenicity or heterogeneous texture correlates with significant pathology independent of measured thickness and should raise suspicion 5.