Artificial shrinkage of blastocysts prior to vitrification improves pregnancy outcome: analysis of 1028 consecutive warming cycles

This study aims to compare implantation, pregnancy, and delivery rates in frozen transfer cycles with blastocysts that were vitrified either with artificial shrinking (AS group) or without (NAS group). Retrospective comparative study of artificial shrinking of blastocysts prior to vitrification and frozen embryo transfer cycles in infertile patients undergoing frozen embryo transfer (FET) was done at the Humanitas Fertility Center between October 2009 and December 2013. Main outcome measure(s) were implantation (IR), pregnancy (PR), and delivery rates (DR) between the two groups. A total of 1028 consecutive warming blastocyst transfer cycles were considered. In 580 cycles (total of 822 blastocysts), artificial shrinking was performed prior to vitrification (AS group), while in the remaining 448 cycles (total of 625 blastocysts), the artificial shrinking was not performed (NAS group). There were no differences in patient age (36.4 +/- 3.7 vs. 36.3 +/- 3.9) and number of embryos transferred (1.41 +/- 0.49 vs. 1.38 +/- 0.50) between groups. The IR, PR, and DR in the AS group were significantly higher (p < 0.05) than in the NAS group (29.9 vs. 23.0 %, 36.3 vs. 27.9 %, and 26.5 vs. 18.1 %, respectively). Performing AS of blastocysts prior to vitrification appears to improve implantation, pregnancy, and delivery rates probably related to a decreased risk of ultrastructural cryodamages, plausible when cryopreserving expanded blastocysts.