An improved protocol for dilution of cryoprotectants from vitrified human blastocysts.

BACKGROUND The purposes of this study were to compare the survival rate of human blastocysts thawed by two different methods after vitrification using electron microscopic (EM) grids, and to report the successful pregnancies and live births resulting from the transfer of blastocysts that had survived the established thawing method. METHODS The blastocysts produced from three pronuclei zygotes in IVF cycles were vitrified on an EM grid. After dilution of cryoprotectant by either a 6- or 2-step method, the survival rate of the blastocysts was compared. RESULTS The survival rate of blastocysts thawed using the 6-step method (82.6%, 100/121) was significantly higher than that of the 2-step method (50.6%, 87/172; P < 0.01). Therefore, we applied the 6-step method in our blastocyst cryopreservation programme. Overall, 34.1% (14/41) of clinical pregnancies and 11 live births were achieved. CONCLUSIONS The present results indicated that a 6-step thawing method was more effective than a 2-step thawing method for thawing vitrified human blastocysts.