Double vitrification and warming does not compromise the chance of live birth after single unbiopsied blastocyst transfer.

study questionDoes double vitrification and thawing of an embryo compromise the chance of live birth after a single blastocyst transfer? SUMMARY ANSWER: The live birth rate (LBR) obtained after double vitrification was comparable to that obtained after single vitrification. WHAT IS KNOWN ALREADY: Double vitrification-warming (DVW) is commonly practiced to accommodate surplus viable embryos suitable for transfer, to allow retesting of inconclusively diagnosed blastocysts in preimplantation genetic testing (PGT), and to circumvent limitations associated with national policies on embryo culture in certain countries. Despite its popularity, the evidence concerning the impact of DVW practice on ART outcomes is limited and lacking credibility. This is the first thorough investigation of clinical pregnancy and LBR following DVW in the case where the first round of vitrification occurred at the zygote stage and the second round occurred at the blastocyst stage in the absence of biopsy. STUDY DESIGN SIZE DURATION: This is a retrospective observational analysis of n = 407 single blastocyst transfers whereby embryos created by IVF/ICSI were vitrified-warmed once (single vitrification-warming (SVW) n = 310) or twice (DVW, n = 97) between January 2017 and December 2021. PARTICIPANTS/MATERIALS SETTING

methodsIn the SVW group, blastocysts were vitrified on Day 5/6 and warmed on the day of embryo transfer (ET). In the DVW group, two pronuclear (2PN) zygotes were first vitrified-warmed and then re-vitrified on Day 5/6 and warmed on the day of ET. Exclusion criteria were ETs from PGT and vitrified-warmed oocyte cycles. All of the ETs were single blastocyst transfers performed at the University Hospital Zurich in Switzerland following natural or artificial endometrial preparation. MAIN RESULTS AND THE ROLE OF CHANCE: The biochemical pregnancy rate, clinical pregnancy rate (CPR), and LBR were all comparable between the DVW and SVW groups. The CPR for DVW was 44.3% and for SVW it was 42.3% ( LIMITATIONS REASONS FOR CAUTION: The study is limited by its retrospective nature. Caution should be taken concerning interpretation of these findings in cases where DVW occurs at different stages of embryo development. WIDER IMPLICATIONS OF THE

findingsThe result of the present study on DVW procedure provides a framework for counselling couples on their chance of clinical pregnancy per warming cycle. It additionally provides confidence and reassurance to laboratory professionals in certain countries where national policies limit embryo culture strategies making DVW inevitable. STUDY FUNDING/COMPETING INTERESTS: This work was supported by the University Research Priority Program 'Human Reproduction Reloaded' of the University of Zurich. The authors have no conflict of interest related to this study to declare. TRIAL REGISTRATION NUMBER: N/A.