title: Cryopreservation of human embryos by vitrification or slow freezing: which one is better?
abstract: OBJECTIVE
  To summarize the available evidence from randomized controlled trials comparing vitrification versus slow freezing for cryopreservation of human embryos.
  
  
  RESULTS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: First Department of Obstetrics and Gynaecology, Unit for Human Reproduction, Medical School, Aristotle University of Thessaloniki, Thessaloniki, Greece. Stratis.Kolibianakis@gmail.com
authors.forename: Efstratios M
authors.initials: EM
authors.lastname: Kolibianakis
authors.forename: Christos A
authors.initials: CA
authors.lastname: Venetis
authors.forename: Basil C
authors.initials: BC
authors.lastname: Tarlatzis
mesh_terms: D015925:Cryopreservation / Q000379:methods*; D046149:Embryo Culture Techniques / Q000379:methods; D004622:Embryo, Mammalian; D019317:Evidence-Based Medicine; D005260:Female; D006801:Humans; D016017:Odds Ratio; D011247:Pregnancy; D018873:Pregnancy Rate; D016032:Randomized Controlled Trials as Topic; D016019:Survival Analysis