The complexity of predicting embryo development potential at the cleavage stages and the emergence of epigenetic risks during prolonged in vitro culture of pre-implantation embryos made it more advantageous to transfer embryos at the morula stage to the uterine cavity. The criteria for estimating embryos at this stage that allow prediction of cryopreservation outcomes have been poorly described. All day 4 embryos (n = 224) were graded 1, 2, 3, 4 or 5 according to blastomere compaction degree (BCD = 100, 75, 50, 25 or 0%, respectively) and the survival and blastocyst formation rate of these morulae were studied after cryopreservation. An inverse dependence was found between survival rate and BCD. Excluded fragments were characterized by low osmotic reaction during exposure to cryoprotective medium and, after freeze-thawing, they were destroyed. As damaged necrotic areas of the embryo can affect their further development rate we proposed blastomeres and biopsy fragments of incomplete compacted morula be removed before embryo cryopreservation. This step led to significant increase in the post-thawing survival rate up to 93.1 ± 4.1%, 75 ± 8.8% and blastocyst formation rate up to 85.2 ± 10.4%, 59.4 ± 5.2% in grade 2 and grade 3 embryos, respectively. There was no significant difference in grade 4 embryos. Therefore the removal of blastomeres and biopsy fragments in incomplete compacted morulae can improve cryopreservation outcomes of grade 2 and grade 3 embryos with BCD.
