Can a female of reproductive age with an endometrium thickness of 9.3 mm on day 11 of her menstrual cycle, currently taking 6mg of unspecified medication, start a new medication for a Frozen Embryo Transfer (FET) cycle?

Can You Start Progesterone for FET with Endometrial Thickness of 9.3mm on Day 11?

Yes, you can start progesterone for the FET cycle with an endometrial thickness of 9.3mm on day 11, as this falls within the optimal range associated with the highest pregnancy success rates in frozen embryo transfer cycles. 1

Optimal Endometrial Thickness for FET

Your patient's endometrial thickness of 9.3mm is ideal for proceeding with progesterone supplementation and frozen embryo transfer:

• The optimal endometrial thickness range for FET cycles is 9-14mm, measured on the day of progesterone supplementation 1
• Endometrial thickness of 9-14mm is associated with significantly higher implantation rates (19% vs 12%), clinical pregnancy rates (30% vs 18%), ongoing pregnancy rates (27% vs 16%), and live birth rates (25% vs 14%) compared to thickness of 7-8mm 1
• These superior outcomes remain statistically significant even after adjusting for confounding variables, with an adjusted odds ratio of 1.83 for clinical pregnancy, 1.8 for ongoing pregnancy, and 1.9 for live birth 1

Timing Considerations

The timing is appropriate for your patient on day 11 of the cycle:

• In medicated FET protocols using 6mg oral estradiol valerate (started on menstrual day 1), progesterone supplementation (800mg daily pessaries) should be initiated when endometrial thickness reaches ≥7mm 1
• Embryo transfer typically occurs 2-3 days after progesterone initiation 1
• Your patient has already achieved the optimal thickness range (9.3mm), so progesterone can be started immediately 1

Evidence Quality and Clinical Implications

• The threshold of 9mm represents a clinically meaningful cutoff, with pregnancy rates increasing gradually from 53% in patients with endometrial thickness <9mm to 77% in those with thickness ≥16mm 2
• Clinical pregnancy and live birth rates increase significantly with increasing endometrial thickness, independent of patient age and embryo quality 2
• However, be aware that some recent critical appraisals suggest that many retrospective studies on endometrial thickness do not adequately address confounding factors, though the preponderance of evidence still supports using these thresholds for clinical decision-making 3

Common Pitfalls to Avoid

• Do not delay progesterone initiation waiting for further endometrial thickening beyond 9mm, as your patient is already in the optimal range 1
• Avoid using endometrial thickness <7mm as a threshold to proceed, as this is associated with significantly lower pregnancy rates 1
• Be cautious with endometrial thickness >14mm, as very thick endometrium (>16mm) may be associated with other considerations, though pregnancy rates remain favorable 2