Vitrification of human embryos has been a paradigm-shifting procedure for higher survival rate compared with the slow freezing protocol. The evidence is scarce to support superior results for vitrifying certain stages of preimplantation embryos. Anecdotal evidence suggests that blastocyst vitrification is more forgiving than cleavage stages. However, data obtained from the procedure of assisted shrinkage of blastocysts before vitrification show a higher survival rate, suggesting that fluid accumulation insides the blastocyst can be a barrier for cryoprotectant to reach the cells. Although reassuring, whether facilitating the cryoprotectants transfer to cells by the laser-assisted shrinkage or other modalities is completely safe remains elusive. Moreover, other claims compare between day-3 embryos vitrification and blastocyst stage, suggesting no difference exists.~One of the most critical stages in embryo development is the maternal to zygotic genome activation (MZA), which occurs at the 4 to 8 cell stages. Therefore, it seems the morula stage is still cleavage but passed the MZA. Morula in the most grading system has compaction for all or the majority of cells so if vitrified, the morula stage can bypass the earlier stage of vitrification as well as the need for the artificial shrinkage for blastocyst stage. Therefore, comparing the vitrification at Day-4 (morula stage) with the blastocyst stage vitrification outcomes with the transfer of all day 5 after warming seems need evaluation. To the best of our knowledge, there has been no random-controlled trial conducted such comparison. Altogether, this trial is to evaluate the morula stage vitrification to blastocyst vitrification on the ongoing pregnancy rate after ICSI.
Comparing the vitrification at Day-4 (morula stage) with the blastocyst stage vitrification outcomes with the transfer of all day 5 after warming seems need evaluation. To the best of our knowledge, there has been no random-controlled trial conducted such comparison. Altogether, this trial is to evaluate the morula stage vitrification to blastocyst vitrification on the ongoing pregnancy rate after ICSI.