Comparison of vitrification and conventional cryopreservation of day 5 and day 6 blastocysts during clinical application

ObjectiveTo evaluate implantation of day 5 and day 6 vitrified and slow-frozen blastocysts.DesignRetrospective analysis comparing two cryopreservation techniques.SettingPrivate IVF clinic.Patient(s)Five hundred eight cryopreserved embryo transfer candidates.Intervention(s)Supernumerary day 5 and day 6 blastocysts were vitrified or slow-frozen and transfered after warming or thawing.Main Outcome Measure(s)Comparison of two cryopreservation techniques with respect to survival rate, implantation, and pregnancy rates of day 5 and day 6 blastocysts.Result(s)In 254 vitrified transfer cycles, survival, embryonic implantation, and clinical pregnancy rates for day 5 blastocysts were 95.9%, 33.4%, 48.7%, respectively, and for day 6 blastocysts 97.5%, 25.9%, 42.8%. In 254 slow-frozen transfer cycles, survival, embryonic implantation, and clinical pregnancy rates for day 5 blastocysts were 91.4%, 29.6%, 42.8%, respectively, and for day 6 blastocysts 94.8%, 28.2%, 43.1%. Overall there was a slightly, but not significantly, higher outcome regarding implantation and clinical pregnancy with the use of day 5 blastocysts (31.3% and 45.4%, respectively) versus day 6 blastocysts (26.7, and 42.9%, respectively).Conclusion(s)Vitrification technique yields the same implantation and pregnancy rate as slow-frozen blastocyst transfers. Slow growing embryos can be cryopreserved on day 6, because they yield a satisfactory survival, implantation, and pregnancy rate.