title: The influence of delayed blastocyst formation on the outcome of frozen-thawed blastocyst transfer: a systematic review and meta-analysis.
abstract: BACKGROUND
  There are conflicting results on whether the rate of blastocyst development before freezing influences the outcome of frozen-thawed blastocyst transfers.
  
  
  METHODS
  We conducted a systematic review and meta-analysis of controlled studies to compare pregnancy outcomes following transfer of thawed blastocysts that were frozen either on Day 5 or Day 6 following fertilization in vitro. Searches were conducted on MEDLINE, EMBASE, Cochrane Library and Web of Science. Study selection and data extraction were conducted independently by two reviewers. The Newcastle-Ottawa Quality Assessment Scale was used for quality assessment.
  
  
  RESULTS
  We identified 15 controlled studies comprising 2502 frozen-thawed transfers involving blastocysts that were either frozen on Day 5 or Day 6. Meta-analysis of these studies showed significantly higher clinical pregnancy rate [relative risk (RR) = 1.14, 95% confidence interval (CI): 1.03-1.26, P = 0.01] and ongoing pregnancy/live birth rate (RR = 1.15, 95% CI: 1.01-1.30, P = 0.03) with Day 5 compared with Day 6 frozen-thawed blastocyst transfers. Sensitivity analysis of those studies where blastocysts frozen on Day 5 or Day 6 were at the same stage of development showed no significant difference in the clinical pregnancy rate (RR = 1.07, 95% CI: 0.87-1.33, P = 0.51) and ongoing pregnancy/live birth rate (RR = 1.08, 95% CI: 0.92-1.27, P = 0.36).
  
  
  CONCLUSIONS
  Slower developing blastocysts cryopreserved on Day 6 but at the same stage of development as those developing to the blastocyst stage on Day 5 have similar clinical pregnancy and ongoing pregnancy/live birth rates following frozen-thawed blastocyst transfers.
authors.affiliation: Guy's and St Thomas' Hospitals NHS Foundation Trust, Assisted Conception Unit, 11th Floor, Tower Wing Guy's Hospital Great Maze Pond, London SE1 9RT, UK. sksunkara@hotmail.com
authors.forename: Sesh Kamal
authors.initials: SK
authors.lastname: Sunkara
authors.forename: Athanasios
authors.initials: A
authors.lastname: Siozos
authors.forename: Virginia Noelle
authors.initials: VN
authors.lastname: Bolton
authors.forename: Yakoub
authors.initials: Y
authors.lastname: Khalaf
authors.forename: Peter Riven
authors.initials: PR
authors.lastname: Braude
authors.forename: Tarek
authors.initials: T
authors.lastname: El-Toukhy
mesh_terms: D000022:Abortion, Spontaneous / Q000453:epidemiology; D001723:Birth Rate; D001755:Blastocyst; D015925:Cryopreservation; D004624:Embryo Transfer; D047108:Embryonic Development; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D011247:Pregnancy; D011256:Pregnancy Outcome; D013997:Time Factors