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

OBJECTIVE: To evaluate implantation of day 5 and day 6 vitrified and slow-frozen blastocysts.
DESIGN: Retrospective analysis comparing two cryopreservation techniques.
SETTING: Private 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.