Endometrial Thickness Target for Starting Progynova in FET Cycles
There is no specific endometrial thickness requirement on day 5 of the cycle to start Progynova (estradiol valerate) for FET preparation—estrogen supplementation typically begins on cycle day 2-3 regardless of baseline endometrial thickness, with the goal of achieving ≥7 mm thickness before progesterone initiation and embryo transfer. 1
Understanding the FET Protocol Timeline
The question appears to conflate two distinct timepoints in FET preparation:
Baseline endometrial thickness (day 2-5 of cycle): This measurement has minimal predictive value for FET success. Research demonstrates no significant relationship between baseline endometrial thickness and clinical pregnancy rates 2. One study found baseline thickness averaged 5.0-5.1 mm on cycle day 3, with no difference between successful and unsuccessful cycles 2.
Target thickness before progesterone initiation: The critical measurement occurs after estrogen priming, not at baseline. Guidelines recommend achieving endometrial thickness ≥7 mm before proceeding to trigger and progesterone supplementation 1, 3.
Evidence-Based Thickness Targets
Minimum Threshold for Transfer
The consensus threshold is ≥7-8 mm measured at the time of progesterone initiation, not at cycle day 5. Multiple studies support this:
- Endometrial thickness <7 mm is associated with significantly lower pregnancy rates 3, 4
- Thickness ≤8 mm correlates with reduced clinical pregnancy rates (24.6% vs 30.3%) and clinical pregnancy rates (18.6% vs 24.0%) compared to >8 mm 5
- Thickness ≤8 mm shows lower live birth rates (23.8%) compared to 9-13 mm (32.2%) or ≥14 mm (34.0%) 6
Optimal Range
Endometrial thickness of 9-13 mm at time of progesterone start is associated with optimal outcomes 6. Thickness >14 mm does not appear to confer additional benefit 6.
Practical Clinical Algorithm
Initiate Progynova on cycle day 2-3 regardless of baseline endometrial thickness 1
Monitor endometrial development with transvaginal ultrasound during estrogen supplementation, typically around cycle day 10-14 3
Assess both thickness and pattern: Target ≥7 mm with trilaminar pattern before progesterone initiation 1, 3
If thickness <7 mm: Continue estrogen supplementation and reassess in 2-3 days. Consider additional interventions if persistently thin 7
Once ≥7 mm achieved: Initiate progesterone supplementation and schedule embryo transfer according to embryo stage 1
Important Caveats
Baseline Thickness Has Limited Predictive Value
Baseline endometrial thickness on cycle day 3-5 does not predict FET success 2. The response to estrogen supplementation (endometrial thickness change) is what matters, not the starting point 2.
BMI Considerations
Higher BMI shows weak positive correlation with baseline endometrial thickness (r = +0.29), but this does not translate to improved outcomes 2. The mechanism appears independent of estradiol levels 2.
Risk Factors for Inadequate Response
Patients at higher risk for thin endometrium despite estrogen supplementation include those with 7:
- Lower BMI
- Nulliparity
- Previous operative hysteroscopy
- Fresh cycle endometrial thickness ≤11.5 mm
For patients with fresh cycle thickness ≤11.5 mm, there is 80% sensitivity for inadequate FET cycle thickness, warranting closer monitoring or alternative protocols 7.
Pattern Matters Beyond Thickness
Evaluate endometrial pattern (trilaminar preferred) in addition to absolute thickness, as pattern independently impacts implantation rates 3.