ABSTRACT
RESEARCH QUESTION: Does the timing of warmed blastocyst transfer in true natural cycle (tNC) differ according to six different commonly used definitions of LH surge, and do differences in timing have any impact on ongoing pregnancy rate (OPR)? DESIGN: Prospective monitoring, including repeated blood sampling and ultrasound analyses of 115 warmed blastocyst transfer cycles performed using tNC between January 2017 and October 2021. RESULTS: The reference timing of follicular collapse +5 days would be equivalent to LH surge +6 days 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 days in the majority of cycles (46.1-69.5%) and less commonly to LH surge +8 days (1.8-38.3%) and +9 days (0-10.4%). For each definition of the LH surge, the OPR were comparable among the different warmed blastocyst transfer timings related to the LH surge (LH surge +6/+7/+8/+9 days). When logistic regression analysis was performed to evaluate the independent effect of variation of warmed blastocyst transfer timing (LH surge +6/+7/+8/+9 days) on OPR and taking LH surge +6 days as the reference, change in timing was not an independent predictor of OPR for any of the definitions of the LH surge. CONCLUSIONS: Employing a policy of performing warmed blastocyst transfer on follicular collapse +5 days and using six different definitions of the LH surge, vitrified-warmed embryo transfer timing is indeed equivalent to LH surge +7/+8 and even +9 days in a significant proportion of tNC with comparable reproductive outcomes.
