Optimal Endometrial Thickness for Frozen Embryo Transfer
For frozen embryo transfer (FET), an endometrial thickness of 9-14 mm measured on the day of progesterone supplementation is associated with optimal pregnancy outcomes.
Understanding Endometrial Thickness in FET Cycles
Optimal Range
- Research shows significantly higher implantation (19% vs. 12%), clinical pregnancy (30% vs. 18%), ongoing pregnancy (27% vs. 16%), and live birth (25% vs. 14%) rates in cycles where endometrial thickness was 9-14 mm compared to 7-8 mm 1
- These differences remained significant after adjusting for confounding variables
Minimum Threshold
- Endometrial thickness should be at least 7 mm before proceeding with embryo transfer
- Pregnancy rates are notably lower when endometrial thickness is <7 mm (7% pregnancy rate) 1
Upper Limit Considerations
- Very thick endometrium (>14 mm) may also be associated with lower pregnancy rates (7%) 1
- This creates a "sweet spot" between 9-14 mm for optimal outcomes
Factors Affecting Endometrial Development
Route of Estrogen Administration
- Transdermal estrogen administration is associated with greater endometrial thickness (9.9 mm) compared to vaginal estrogen (9.3 mm) 2
- Transdermal route also shows shorter treatment duration (13.6 vs 15.5 days) and higher patient satisfaction 2
Patient-Related Factors
Several factors may predict inadequate endometrial thickness (<8 mm) in FET cycles:
- Lower body mass index
- Nulliparity
- Previous operative hysteroscopy
- Thinner endometrial lining in previous fresh IVF cycles 3
Previous Fresh Cycle as Predictor
- A maximum thickness of 11.5 mm or less in a fresh cycle was 80% sensitive and 70% specific for inadequate frozen cycle thickness 3
- This can help identify patients who may need additional interventions to achieve adequate endometrial thickness
Clinical Recommendations
Monitoring Protocol
- Begin endometrial preparation with estrogen supplementation
- Monitor endometrial thickness via transvaginal ultrasound
- Target a thickness of 9-14 mm before starting progesterone supplementation
- Proceed with embryo transfer 2-3 days after progesterone initiation
When Thickness is Suboptimal
- For patients with endometrial thickness <7 mm, consider:
- Changing estrogen administration route (transdermal may be more effective) 2
- Extended estrogen supplementation
- Cancellation of cycle if thickness remains inadequate
Important Caveats
- According to the 2024 ESHRE guidelines, the decision to perform double embryo transfer instead of single embryo transfer should not be based on endometrial characteristics 4
- Some studies suggest baseline endometrial thickness or endometrial thickness change in response to estrogen may not be predictive of FET success 5
- However, the weight of evidence supports targeting the 9-14 mm range for optimal outcomes
Practical Considerations
- Transvaginal ultrasound is the preferred method for measuring endometrial thickness
- Measurements should be taken in the sagittal plane at the thickest part of the endometrium
- Both endometrial thickness and quality (trilaminar pattern) should be assessed