Outcome of closed blastocyst vitrification in relation to blastocyst quality: evaluation of 759 warming cycles in a single-embryo transfer policy

BACKGROUND: In order to optimize blastocyst cryopreservation, vitrification was introduced as the routine procedure instead of slow freezing. The outcome of a closed blastocyst vitrification system was evaluated in relation to the blastocyst score before cryopreservation in single embryo transfers (SETs). METHODS: Supernumerary blastocysts of IVF/ICSI patients with a fresh Day 5 transfer were vitrified using CBS-VIT High Security (HS) straws. In 759 warming cycles, morphological survival and transfer rates were assessed in relation to the blastocyst score and the day of vitrification. Pregnancy rates were assessed in 530 SET and 156 double embryo transfer (DET) cycles. Implantation rates per embryo transferred in SET cycles were analysed according to blastocyst quality and day of cryopreservation. RESULTS: Immediate morphological survival was 77.8% (921/1185) and the transfer rate per warmed blastocyst was 70.7% (838/1185). Survival rates were higher for Day 5 early blastocysts (86.7%) compared with full (78.7%) or expanded blastocysts (72.7%). A reduced survival rate of 70.1% was found for Day 6 blastocysts compared with Day 5 blastocysts (80.6%, P < 0.001). The overall clinical/ongoing pregnancy rate after SET was 16.4/14.2% and 24.4/20.5% after DET with an ongoing multiple pregnancy rate of 21.8% (7/32) after DET. Significantly lower implantation rates were found for Day 5 early blastocysts (10.6%) compared with advanced blastocysts (17.5%, P < 0.05). Similar implantation potentials for Day 5 and 6 blastocysts (14.3 versus 13.7%) were found. CONCLUSIONS: Successful cryopreservation of blastocysts from the early cavitating up to expanded blastocyst stages is possible using a closed HS device. The choice between single or double frozen blastocyst transfer should depend on blastocyst expansion after vitrification.