Comparison of post-thaw/warming survival rates, reexpansion, and pregnancy outcomes between vitrification and slow-freezing methods for human blastocyst cryopreservation

Blastocysts were cryopreserved with slow-freezing (SF) before May 2007 and with vitrification (VT, cryotop) technique since then. Cryopreserved embryos were thawed/warmed and cultured for 2 to 6 hours before transfer. In total, 201 consecutive frozen blastocyst transfer (FBT) cycles (129 SF and 72 VT) were analyzed. The survival rate for VT (98.8%) was significantly higher than SF (93.6%, p=0.022). There was also a higher percentage of reexpanded blastocysts for VT embryos (85.6%) than for SF embryos (67.0%, p=0.0001). However, when surviving embryos were transferred implantation rate (VT 38.3% vs SF 33.5%, p=0.37) and ongoing PR (VT 52.8% vs SF 54.3%, p=0.95) were similar. These results indicate that embryo survival was better with VT but the quality of surviving embryos and implantation potential were similar.