The worldwide frozen embryo reservoir: methodologies to achieve optimal results.

Cryopreservation of the human embryo has been successfully achieved at the zygote (day 1), cleavage (day 2/3), and blastocyst (day 5) stages; however, each stage presents specific advantages and disadvantages. During the past decades, two major methods have been applied: slow freezing (equilibrium procedure) and vitrification (nonequilibrium procedure). The overwhelming majority of published data prove that the latest vitrification methods induce less cellular trauma and are a more effective cryopreservation technique of human embryos than any other versions of slow freezing. For this reason, fragmented and slow-cleaving embryos that normally would not be recommended may be revaluated for cryopreservation by using the vitrification method. Furthermore, if laser-assisted necrotic blastomere removal is associated with the slow-freezing/thawing procedure, good clinical results can be obtained. Finally, the most proper embryo cleavage stage at which to perform cryopreservation has to be assessed according to clinical indications and laboratory experience.