title: Clinical pregnancy rate following frozen embryo transfer is higher with blastocysts vitrified on day 5 than on day 6.
abstract: OBJECTIVE
  The aim of this study was to compare the pregnancy rates between good quality blastocysts vitrified on day 6 versus blastocysts vitrified on day 5 after fertilization.
  
  
  METHODS
  This is a retrospective cohort study of 791 freeze-thaw cycles of blastocysts vitrified either on day 5 or on day 6 and transferred between January 2012 and October 2015. Five hundred and thirty-seven cycles included blastocysts vitrified on day 5, and 254 cycles included blastocysts vitrified on day 6.
  
  
  RESULTS
  The age of the patients and the proportion of embryos that survived the thawing process were comparable between the two groups. More good quality embryos were transferred in the group in which blastocysts were vitrified on day 6 (1.2 vs. 1.3, p = 0.005), but the clinical pregnancy rate (44 vs. 33 %, p = 0.002) and the ongoing pregnancy rate (41 vs. 28 %, p < 0.001) were higher in the group in which blastocysts were vitrified on day 5. Multivariate regression analysis adjusting for patient's age, number of good quality embryos transferred (≥3BB), and treatment protocol demonstrated that the day 6 vitrified group had a significantly lower clinical pregnancy rate compared to the day 5 vitrified group (OR 0.54, 95 % CI 0.38-0.76).
  
  
  CONCLUSIONS
  The clinical pregnancy rate following frozen embryo transfer is significantly lower with blastocysts vitrified on day 6 compared to blastocysts vitrified on day 5.
authors.affiliation: TRIO Fertility Partners, Division of Reproductive Sciences, University of Toronto, 655 Bay St 11th floor, Toronto, ON, Canada, M5G 2K4. jigalh@hotmail.com.|Department of Obstetrics and Gynecology, Chaim Sheba Medical Centre, Tel-Hashomer, Ramat Gan, Israel. jigalh@hotmail.com.
authors.forename: Jigal
authors.initials: J
authors.lastname: Haas
authors.affiliation: TRIO Fertility Partners, Division of Reproductive Sciences, University of Toronto, 655 Bay St 11th floor, Toronto, ON, Canada, M5G 2K4.
authors.forename: Jim
authors.initials: J
authors.lastname: Meriano
authors.affiliation: TRIO Fertility Partners, Division of Reproductive Sciences, University of Toronto, 655 Bay St 11th floor, Toronto, ON, Canada, M5G 2K4.
authors.forename: Carl
authors.initials: C
authors.lastname: Laskin
authors.affiliation: TRIO Fertility Partners, Division of Reproductive Sciences, University of Toronto, 655 Bay St 11th floor, Toronto, ON, Canada, M5G 2K4.
authors.forename: Yaakov
authors.initials: Y
authors.lastname: Bentov
authors.affiliation: Department of Obstetrics and Gynecology, Chaim Sheba Medical Centre, Tel-Hashomer, Ramat Gan, Israel.
authors.forename: Eran
authors.initials: E
authors.lastname: Barzilay
authors.affiliation: TRIO Fertility Partners, Division of Reproductive Sciences, University of Toronto, 655 Bay St 11th floor, Toronto, ON, Canada, M5G 2K4.
authors.forename: Robert F
authors.initials: RF
authors.lastname: Casper
authors.affiliation: TRIO Fertility Partners, Division of Reproductive Sciences, University of Toronto, 655 Bay St 11th floor, Toronto, ON, Canada, M5G 2K4.
authors.forename: Ken
authors.initials: K
authors.lastname: Cadesky
mesh_terms: D000328:Adult; D001755:Blastocyst; D015925:Cryopreservation; D010064:Embryo Implantation / Q000502:physiology; D004624:Embryo Transfer / Q000379:methods*; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D011247:Pregnancy; D018873:Pregnancy Rate; D058989:Vitrification
keywords: Blastocyst day 6; Frozen embryo transfer; Vitrification