This multi-centre study evaluated systematically the influence of the duration of cryostorage on the outcome of thawing cycles when using slow-frozen oocytes. The thawing cycles were retrospectively divided into three main groups based on cryostorage duration: group A, 1-3 months; group B, 4-6 months; and group C, 7-48 months. Group C was subsequently divided into three subgroups: group C1, 7-9 months; group C2, 10-12 months; and group C3, 13-48 months. Main outcome measures observed were oocyte survival after thawing, fertilization, cleavage; embryo quality and development, implantation, and birth. No significant differences in main outcome measures were observed between all the groups studied. In conclusion, human oocytes can be safely cryostored for several years. This finding could encourage the wider use of oocyte cryopreservation as a clinical procedure in assisted reproduction.

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