title: Comparison of the clinical outcomes of day 4 and 5 embryo transfer cycles.
abstract: OBJECTIVE
  The majority of embryo transfers (ETs) to date have been performed on day 3 to reduce the potential risk of developmental arrest of in vitro cultured embryos before ET. Development of sequential media has significantly improved culture conditions and allowed blastocyst transfer on day 5. While day 5 ET provides higher clinical pregnancy outcomes with reduced risks of multiple pregnancies, it still has potential risks of developmental arrest of IVF embryos. The aim of this study was to evaluate the clinical outcomes of day 4 ETs and compare the efficacy of day 4 ET with day 5 ET.
  
  
  METHODS
  From 2006 to 2009, a total of 747 fresh IVF-ET cycles were retrospectively analyzed (day 4, n=440 or and day 5, n=307). The cycles with any genetic factors were excluded. The rates of matured oocytes, fertilization, good embryos, and clinical pregnancy of the two groups were compared. The chi-square test and t-test were used for statistical analysis.
  
  
  RESULTS
  There were no significant differences between the two groups with respect to the mean age of the females and rates of matured oocytes. The pregnancy outcomes of day 4 ET (40.7%) were similar to those of day 5 ET (44.6%). The implantation rate of day 5 ET (24.2%) was significantly higher than that of day 4 ET (18.4%) (p=0.003).
  
  
  CONCLUSIONS
  Day 4 ET can be chosen to avoid ET cancellation in day 5 ET resulting from suboptimal circumstances in the IVF laboratory, but the decremented quality of embryos for transfer and the decreased pregnancy rate must be taken into consideration.
authors.affiliation: Laboratory of Reproductive Biology and Infertility, Cheil General Hospital and Women's Healthcare Center, Kwandong University College of Medicine, Seoul, Korea.
authors.forename: Sun-Hee
authors.initials: SH
authors.lastname: Lee
authors.forename: Hyoung-Song
authors.initials: HS
authors.lastname: Lee
authors.forename: Chun Kyu
authors.initials: CK
authors.lastname: Lim
authors.forename: Yong-Seog
authors.initials: YS
authors.lastname: Park
authors.forename: Kwang Moon
authors.initials: KM
authors.lastname: Yang
authors.forename: Dong Wook
authors.initials: DW
authors.lastname: Park
keywords: Blastocyst; Embryo culture; Embryo selection; Embryo transfer; Embryo-uterine synchrony; Embryonic grading; Pregnancy outcome