title: A comparison of day 5 and day 6 blastocyst transfers.
abstract: OBJECTIVE
  To compare implantation and pregnancy rates according to the day of embryo transfer (day 5 or 6 after oocyte retrieval) when transfer was postponed until expanded blastocysts developed.
  
  
  METHODS
  Retrospective clinical study.
  
  
  METHODS
  Private ART center.
  
  
  METHODS
  One-hundred and eighty-three women undergoing blastocyst-stage embryo transfer following in vitro fertilization.
  
  
  METHODS
  Bipronucleate oocytes were grown for up to 144 hours and subsequently transferred only when at least one embryo attained the expanded blastocyst stage.
  
  
  METHODS
  Implantation and pregnancy rates.
  
  
  RESULTS
  Blastocysts transferred on day 5 implanted at nearly twice the rate of blastocysts transferred on day 6 (36.3% vs. 19.0%). Pregnancy rates were also almost twice as high among the day 5 transfer patients (59.3% vs. 32.3%). In addition, more blastocysts developed (3.6 vs. 2.4), and more were transferred (2.7 vs. 2.3) to the day 5 transfer patients, although the proportion of expanded blastocysts among the blastocysts that were transferred was the same for the two groups (91.7% vs. 93.6%).
  
  
  CONCLUSIONS
  Embryos that develop to the expanded blastocyst stage and are transferred on day 5 after retrieval are approximately twice as likely to implant compared to those for which expansion and transfer are delayed until day 6.
authors.affiliation: Fertility Center of Las Vegas, Las Vegas, Nevada 89117, USA.
authors.forename: B S
authors.initials: BS
authors.lastname: Shapiro
authors.forename: K S
authors.initials: KS
authors.lastname: Richter
authors.forename: D C
authors.initials: DC
authors.lastname: Harris
authors.forename: S T
authors.initials: ST
authors.lastname: Daneshmand
mesh_terms: D000328:Adult; D001755:Blastocyst; D010064:Embryo Implantation; D004624:Embryo Transfer; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D011247:Pregnancy; D018873:Pregnancy Rate; D012189:Retrospective Studies; D013997:Time Factors