Timing of vitrified-warmed blastocyst transfer in true natural cycle according to different definitions of luteinizing hormone surge in the literature

Pinpointing the day of ovulation, usually by documentation of the luteinizing hormone (LH) surge, is crucial for timing of frozen-embryo transfer (FET) in true natural cycle (t-NC) to maximize reproductive success; however, there is no consensus on the definition of the LH surge. Although the usual practice is to schedule warmed blastocyst transfer on LH surge +6 day, only three retrospective studies explored the impact of timing related to the LH surge in t-NC on reproductive outcomes with contradictory results. On this basis, we queried our database of t-NC FET cycles to determine the impact of different definitions of LH surge, as commonly used in the existing literature, on the date of warmed blastocyst transfer. Additionally, we compared the ongoing pregnancy rates (OPRs) after t-NC FET according to different definitions of the LH surge. Monitoring with repeated blood sampling and ultrasound analyses of 115 warmed blastocyst transfer cycles performed in t-NC during January 2017 – October 2021, were included. Our reference timing of follicular collapse +5 day would be equivalent to LH surge +6 day in only 5.2-41.2% of the cycles employing the six different definitions of the LH surge. In contrast, the reference timing was equivalent to LH surge +7 day in the majority of cycles (46.1-69.5%) and less commonly to LH surge +8 day (1.8-38.3%) and +9 day (0-10.4%). Importantly, for each different definition of the LH surge, the OPRs were comparable among the LH surge +6/+7/+8/+9 timings, reflecting the high degree of flexibility of the window of implantation.