title: Cryopreservation of human embryos by vitrification or slow freezing: a systematic review and meta-analysis.
abstract: 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?
  
  
  METHODS
  Systematic review and meta-analysis.
  
  
  METHODS
  University-based hospital.
  
  
  METHODS
  Not applicable.
  
  
  METHODS
  Vitrification versus slow freezing for cryopreservation of human embryos.
  
  
  METHODS
  Postthawing survival rate.
  
  
  RESULTS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Unit for Human Reproduction, 1st Department of Obstetrics and Gynaecology, Papageorgiou General Hospital, Thessaloniki, Greece.
authors.forename: Kalliopi E
authors.initials: KE
authors.lastname: Loutradi
authors.forename: Efstratios M
authors.initials: EM
authors.lastname: Kolibianakis
authors.forename: Christos A
authors.initials: CA
authors.lastname: Venetis
authors.forename: Evangelos G
authors.initials: EG
authors.lastname: Papanikolaou
authors.forename: George
authors.initials: G
authors.lastname: Pados
authors.forename: Ioannis
authors.initials: I
authors.lastname: Bontis
authors.forename: Basil C
authors.initials: BC
authors.lastname: Tarlatzis
mesh_terms: D001755:Blastocyst; D002470:Cell Survival; D002970:Cleavage Stage, Ovum; D015925:Cryopreservation / Q000379:methods*; D003451:Cryoprotective Agents / Q000009:adverse effects; D010064:Embryo Implantation; D004624:Embryo Transfer; D004622:Embryo, Mammalian; D019317:Evidence-Based Medicine; D005260:Female; D005307:Fertilization in Vitro; D005615:Freezing; D006801:Humans; D050498:Live Birth; D016017:Odds Ratio; D011247:Pregnancy; D018873:Pregnancy Rate; D018570:Risk Assessment; D013997:Time Factors
chemical_list: D003451:Cryoprotective Agents