The window is wide: flexible timing for vitrified-warmed embryo transfer in natural cycles.

Research Question: What is the optimal timing for transfer in natural cycle vitrified-warmed embryo transfers (NC-VET)?

Design: This retrospective cohort study uses data from a large university-affiliated IVF clinic. The study included 341 NC-VET cycles with autologous oocytes and non-preimplantation genetic testing, vitrified embryos from January 2013 to September 2017. Each cycle was classified by timing of embryo transfer in relation to LH surge ≥20 IU/l. Group 1: LH ≥20 IU/l one day and blastocyst was transferred 6 days later; Group 2: LH ≥20 IU/l two consecutive days and blastocyst was transferred 6 days after the initial surge; Group 3: LH ≥20 IU/l two consecutive days and blastocyst was transferred 7 days after the initial surge. The primary outcome was ongoing pregnancy rate (OPR). The secondary objective was to compare OPR in relation to serum oestradiol dynamics and progesterone concentration (according to threshold 1.0 ng/ml) 6 days prior to embryo transfer.

Results: OPR were similar for all three groups (66.8%, 65.0%, 62.9% for Groups 1, 2 and 3, respectively). When stratified according to oestradiol and progesterone, no significant differences were noted in OPR.

Conclusions: The results suggest that the timing of blastocyst transfer in a natural cycle after LH surge is flexible within 24 h. Outcomes are equally good with day of embryo transfer 6 or 7 days after LH surge date. Oestradiol dynamics and progesterone concentration 6 days prior to NC-VET did not have a significant impact on OPR.