Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis

Objective: To examine the literature systematically in order to identify prospective comparative trials answering the following question: Is vitrification of human embryos associated with a higher postthawing survival rate as compared with slow freezing? Design: Systematic review and meta-analysis. Setting: University-based hospital. Patient(S): Not applicable. Intervention(s): Vitrification versus slow freezing for cryopreservation of human embryos. Main Outcome Measure(s): Postthawing survival rate. Result(s): Four eligible studies were identified, three of which were randomized controlled trials. Overall, the current review summarizes information from 8,824 cryopreserved human cleavage stage embryos/blastocysts (vitrification: n = 7,482; slow freezing: n = 1,342). Survival rate of cleavage stage embryos was significantly higher after vitrification as compared with slow freezing (odds ratio 15.57, 95% confidence interval 3.68-65.82; random effects model). Postthawing survival rate of vitrified blastocysts was significantly higher compared with that observed with slow freezing (odds ratio 2.20, 95% confidence interval 1.53-3.16; fixed effects model). Conclusion(s): Vitrification appears to be associated with a significantly higher postthawing survival rate than slow freezing. Further prospective trials are necessary to confirm the above results and, in addition, allow the evaluation of-the two cryopreservation methods in terms of pregnancy achievement.