Vitrified-warmed blastocyst transfer timing related to LH surge in true natural cycle and its impact on ongoing pregnancy rates.

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.