The Embryologist's Impact on Blastocyst Vitrification and Thawing

The goal of this monocentric retrospective observational study is to analyse, for the first time in literature, the role of the embryologist who freezes and thaws the embryos. The primary objective of the study is to understand whether the embryologist who freezes and thaws the embryo influences the CPR (clinical pregnancy rate). Secondary objectives, in case of statistically significant influence, are: * Evaluate who influences more the CPR, between the embryologist who freezes the embryos and the embryologist who thaws the embryos. * Evaluate if the embryologist who freezes and thaws the embryo impacts more than the embryologist or the physician who performs the ET. * Evaluate whether the embryologists improve their performances, as their experience increases. * Evaluate, through an external validation test, whether the model used can also be applied at other PMA centres The study will consider all the freezing (vitrifications) and thawing procedures, performed at Humanitas Fertility Center between January 2019 and June 2023. The study will include the transfers of single blastocysts, cryopreserved at Humanitas Fertility Center. On the other hand, the blastocysts transferred from other centres and the donor blastocysts will be excluded from the study, as well as multiple blastocysts transfers and the LP cycles. The embryologist's experience will be assessed in terms of number of previous cryopreservation (vitrification) and thawing procedures. We will include in the study all the embryologists who performed at least 50 freezing and thawing. For the evaluation of the study objectives, data will be gathered using a specific internal web-based database. The final model will be created by analysing the Humanitas Fertility Center dataset and validated using datasets extrapolated from similar population from the San Raffaele Hospital infertility centre