Endometrial compaction before frozen euploid embryo transfer improves ongoing pregnancy rates

Objective: To assess whether the calculated difference in endometrial thickness from the end of the estrogen phase to the day of ET (after 6 days of P in hormonally prepared cycles) is associated with ongoing pregnancy rates in euploid frozen ETs (FETs). Design: An observational cohort study. Setting: Single tertiary care medical center. Patient(s): Ultrasound images from 234 hormonally prepared FET cycles were assessed. All the transfers were elective single ETs of a euploid embryo, post–preimplantation genetic testing for aneuploidy (PGT-A). Intervention(s): Ultrasound measurements of peak endometrial thickness at the end of the estrogen phase and again after 6 days of P at the time of ET. Main Outcome Measure(s): Ongoing pregnancy rate in relation to the delta between endometrial thickness at the end of estrogen phase and at the time of ET. Result(s): We calculated the ongoing pregnancy rate in cycles where the endometrial lining decreased (compacted) after addition of P by 5%, 10%, 15%, and 20% and demonstrated a significantly higher pregnancy rate after all rates of compaction of the endometrial lining in comparison with cycles where the endometrial lining did not compact. The ongoing pregnancy rate in this cohort, after compaction of 15% or more, was 51.5%, compared with 30.2% in cycles where the endometrial lining did not compact. Conclusion(s): There is a significant correlation between endometrial lining compaction and ongoing pregnancy rate in FET cycles of euploid embryos. These findings help to explain why some euploid embryos may fail to implant.