Dh. Edgar et al., The influence of prefreeze growth rate and blastomere number on cryosurvival and subsequent implantation of human embryos, J AS REPROD, 18(3), 2001, pp. 135-138
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.