title: Slow controlled-rate freezing of sequentially cultured human blastocysts: an evaluation of two freezing strategies.
abstract: BACKGROUND
  To optimize blastocyst cryopreservation, the prerequisite is to develop a better understanding of factors that influence their survival and implantation potential. Therefore, the aim of the present work was to evaluate, retrospectively, the outcome of blastocyst cryopreservation in a day 2/3 fresh embryo transfer programme.
  
  
  METHODS
  Two different freezing strategies were compared: a first strategy (strategy A: 3007 blastocysts frozen) consisted of freezing those blastocysts that had at least a cavity; a second strategy (strategy B: 3831 blastocysts frozen) consisted of freezing only more advanced stage blastocysts with a good quality inner cell mass and trophectoderm. The outcome of cryopreservation, as related to the two different freezing strategies, was analysed. In addition, after freezing and thawing, we evaluated the influence of blastocyst developmental characteristics on immediate morphological survival and further development in vitro.
  
  
  RESULTS
  The immediate morphological survival after thawing was higher for early blastocysts as compared to advanced and hatching blastocysts. The further developmental potential in vitro of thawed blastocysts was higher for advanced and hatching blastocysts as compared to early blastocysts. As a result, the percentage of deliveries, calculated as a percentage of started thawing cycle, and the percentage of children born, calculated as a percentage of embryos transferred, was not different for strategies A and B.
  
  
  CONCLUSIONS
  The results clearly indicate that culture conditions and cryopreservation procedures of blastocysts need to be further improved.
authors.affiliation: Centre for Reproductive Medicine, Academic Hospital, Dutch-speaking Brussels Free University, Belgium. Etienne.vandenabbeel@az.vub.ac.be
authors.forename: Etienne
authors.initials: E
authors.lastname: Van den Abbeel
authors.forename: Michel
authors.initials: M
authors.lastname: Camus
authors.forename: Greta
authors.initials: G
authors.lastname: Verheyen
authors.forename: Linda
authors.initials: L
authors.lastname: Van Waesberghe
authors.forename: Paul
authors.initials: P
authors.lastname: Devroey
authors.forename: André
authors.initials: A
authors.lastname: Van Steirteghem
mesh_terms: D001755:Blastocyst / Q000166:cytology / Q000378:metabolism / Q000473:pathology*; D003080:Cold Temperature; D015925:Cryopreservation / Q000379:methods*; D003451:Cryoprotective Agents / Q000494:pharmacology; D046149:Embryo Culture Techniques; D010064:Embryo Implantation; D004624:Embryo Transfer; D005260:Female; D005307:Fertilization in Vitro / Q000379:methods*; D005615:Freezing; D006801:Humans; D009865:Oocytes / Q000473:pathology; D013997:Time Factors
chemical_list: D003451:Cryoprotective Agents