Does duration of cryostorage affect survival rate, pregnancy, and neonatal outcomes? Large‐scale single‐center study of slush nitrogen (SN 2 ) vitrified‐warmed blastocysts

OBJECTIVE To evaluate the effects of the duration of cryostorage on clinical outcomes after embryo transfer of vitrified blastocysts stored in an open-device slush-nitrogen (SN2 ) system. METHODS A retrospective cohort study was carried out on 1632 autologous vitrified-warmed blastocyst transfer cycles between January 2013 and June 2014. Duration of cryostorage was divided into four groups: Group I: 0-6 months (n=937); Group II: 7-12 months (n=299); Group III: 13-24 months (n=165); and Group IV: ≥25 months (n=231). The effects of the duration of cryostorage on the survival rate (SR), clinical pregnancy rate (CPR), live birth rate (LBR), and neonatal outcomes of vitrified blastocysts stored in an open-device SN2 system were evaluated. RESULTS There were no significant differences between groups in SR, CPR, LBR, and neonatal outcomes after autologous vitrified-warmed blastocyst transfer. Multivariate logistic regression analysis showed no effect on LBR from duration of cryostorage. CONCLUSION Vitrification using SN2 and long-term cryostorage in an open-device system are safe and effective and do not significantly affect clinical outcomes after embryo transfer.