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 cryop reserved Day 2 embryos with only 2 blastomeres can be increased as a conseq uence of increasing their blastomere content by extending the prefreeze cul ture 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 c ryopreserved. 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 di scarded. Thawed embryos from each category were assessed for survival, post thaw cleavage, and implantation. Results: The proportion of thawed embryos that survived, the proportion of surviving embryos that underwent postthaw cleavage, and the implantation ra te of transferred embryos were all reduced in the slower growing cryopreser ved embryos. Conclusions: The growth rate, and not the number of blastomeres per se, is a critical factor in predicting the developmental potential of cryopreserve d embryos.