Cryopreservation of human embryos by vitrification or slow freezing: which one is better?

PURPOSE OF REVIEW: To summarize the available evidence from randomized controlled trials comparing vitrification versus slow freezing for cryopreservation of human embryos. RECENT FINDINGS: Vitrification, as compared with slow freezing, appears to be better in terms of postthawing survival rates both for cleavage-stage embryos odds ratio (OR): 6.35, 95% confidence interval (CI): 1.14-35.26, random effects model and for blastocysts (OR: 4.09, 95% CI: 2.45-6.84, random effects model). Furthermore, postthawing blastocyst development of embryos cryopreserved in the cleavage stage is significantly higher with vitrification as compared with slow freezing (OR: 1.56, 95% CI: 1.07-2.27, fixed effects model). No significant difference in clinical pregnancy rates per transfer could be detected between the two cryopreservation methods (OR: 1.66, 95% CI: 0.98-2.79). SUMMARY: Currently, vitrification does not appear to be associated with an increased probability of pregnancy. However, a significant advantage of vitrification over slow freezing in terms of postthawing survival rates is present for embryos cryopreserved both at the cleavage and at the blastocyst stages. The above conclusions are based on limited data, and thus further properly designed randomized controlled trials are needed.