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.