Received April 12, 2011
Accepted April 22, 2011

Objective: To compare the results of a program of in vitro fertilization with embryos cryopreserved at blastocyst stage, either by vitrification or slow cooling.Patients: transfer cycles at blastocyst stage were considered during the years 2009 and 2010. Methods: From our database we selected the cycles corresponding to the above characteristics. The results were analyzed using Graph Pad InStat, using the Fisher exact test. Results: There were 133 cycles corresponding to the transfer of cryopreserved embryos in blastocyst stage, 107 cycles corresponding to the slow technique (grL) and the remaining 26 blastocysts were vitrified (grV). In the grL group there were 3.28±1.18 thawed blastocysts per cycle and the grV group had 2.96±1.77 with an average of 2.35±0.90 (grL) and 2.30±0 97 (grV), respectively. The difference in the recovery of viable blastocysts between the two groups was not significant (P = 0.4315). In grL 11 were due to suspend transfers (10.3%) while the grV 3 (11.5%) due to lack of viability of embryos recovered. In grL there were 39 (40.6%) clinical pregnancies versus 13 in the GRV 13 (56.5%). The observed difference was not significant (P = 0.2416). Conclusions: Slow Cryopreservation at blastocyst stage is used at our institution with satisfactory results. In the last two years we started to explore the possible advantages of vitrification to determine whether it would justify changing the slow protocol in use. While the results show a tendency to be better with vitrification, the number of cases studied does not yet allow statistically valid conclusions.