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Vitrification vs. slow cooling protocol using embryos cryopreserved in the 5th or 6th day after oocyte retrieval and IVF outcomes.

Modifying cryopreservation protocols may be seen as a way to simplify cryobanking procedure and increase satisfying outcomes. The aim of the study was to evaluate the influence of slow cooling protocol and vitrification on IVF outcomes using embryos preserved in the 5th or 6th day after oocyte retrieval. The study compared 2 groups of human embryos underwent slow cooling protocol (n=189) and vitrification (n=58). All embryos were cryopreserved in the 5th or 6th day after oocyte retrieval. Pre- and postfreezing embryo evaluation was performed in 2 or 3 steps scale, respectively. The study evaluates the effectiveness of two freezing methods and influence of the freezing day, pre- and postfreezing embryo grading on clinical pregnancy rate. Study showed higher pregnancy rate after vitrification (50.4%) than slow cooling protocol (25.9%). Significantly higher pregnancy rate was observed, when embryo preserved in the 5th day after oocyte retrieval (50.3%) than in the 6th day (22.7%). Postfreezing embryos evaluation showed that high quality blastocysts gave nearly four times better pregnancy outcomes than the ones evaluated as poor quality, and three times better than the ones evaluated as moderate. Prospective trials are needed to evaluate pregnancy and neonatal outcomes after vitrification. The number of controlled studies concerning vitrification has not been large enough, yet.