Recent advances in culture systems have made it possible to develop IVF embryos into blastocysts (BLs) easily, and BL transfer has proven effective for increasing the pregnancy rate. Therefore, cryopreservation of BLs has become an important technique. However, the cryopreservation of human BLs using both slow freezing method and vitrification with straws resulted in disappointing survival rates. An innovative strategy to circumvent the problem of intracellular ice formation in less permeating embryos is ultra-rapid vitrification, in which embryos are vitrified with a minimal volume of a vitrification solution using minute tools such as cryoloop. Since we started this vitrified BL program using cryoloop, 229 clinical pregnancies (44.1%) and 291 implantations (29.0%) out of 519 BLs transferred cycles were obtained. 147 babies were born in 108 deliveries. Miscarriage rate is 22.7% (52/229). We have applied assisted hatching (AHA) of warmed BLs in order to improve the implantation and pregnancy rates, because cryopreservation including vitrification induces hardening of the Zona pellucida (ZP), which may impair spontaneous hatching after warming. Moreover, we have also applied artificial shrinkage (AS) of the blastoceoleic cavity of expanded or larger stage of BLs in order to increase the survival rate, because the results of our vitrified BL program showed that the efficiency of the vitrification method was dependent on the stage of BL development and was negatively correlated with the size of the blastocoele.
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Vol. 22 • No. 2