CLINICAL ASSISTED REPRODUCTION: The Influence of Prefreeze Growth Rate and Blastomere Number on Cryosurvival and Subsequent Implantation of Human Embryos

Purpose: To determine whether the relatively low implantation rate of cryopreserved Day 2 embryos with only 2 blastomeres can be increased as a consequence of increasing their blastomere content by extending the prefreeze culture time. Methods: Of a total of 3480 Day 2 embryos studied, 1921 (55.2%) had reached the 4-cell stage by 40 h postinsemination (FAST) and were transferred or cryopreserved. The remaining embryos that underwent subsequent cell division by 46 h (INTERMEDIATE; 18.3% of total) or 66 h (SLOW; 20.3% of total) were also cryopreserved whereas the 6.2% that remained arrested at 66 h were discarded. Thawed embryos from each category were assessed for survival, postthaw cleavage, and implantation. Results: The proportion of thawed embryos that survived, the proportion of surviving embryos that underwent postthaw cleavage, and the implantation rate of transferred embryos were all reduced in the slower growing cryopreserved embryos. Conclusions: The growth rate, and not the number of blastomeres per se, is a critical factor in predicting the developmental potential of cryopreserved embryos.