title: Comparison of clinical outcomes between single and double vitrified-warmed blastocyst embryo transfer according to the day of vitrification.
abstract: OBJECTIVE
  To compare the efficacy of single vitrified-warmed blastocyst embryo transfer (SVBT) versus double vitrified-warmed blastocyst embryo transfer (DVBT) according to the day of vitrification.
  
  
  METHODS
  This retrospective study included a total of 1,051 cycles in women less than 37 years of age with their autologous SVBT cryopreserved on day 5 (5d-SVBT, n = 737) or day 6 (6d-SVBT, n = 154) and DVBT on day 5 (5d-DVBT, n = 129) or day 6 (6d-DVBT, n = 31) from January 2009 to December 2011.
  
  
  RESULTS
  The clinical pregnancy rate (41.8 % vs. 48.1 %, p = 0.184) and ongoing pregnancy rate (36.6 % vs. 45.0 %, p = 0.072) were not significantly different between the 5d-SVBT group and the 5d-DVBT group. However, the clinical pregnancy (29.9 % vs. 58.1 %, p = 0.003) and ongoing pregnancy rates (23.4 % vs. 51.6 %, p = 0.001) were significantly lower in the 6d-SVBT group compared with those in the 6d-DVBT group. The implantation rate (42.2 % vs. 34.5 %, p = 0.03) of the 5d-SVBT group was significantly higher than that of the 5d-DVBT group, while the implantation rate (29.9 % vs. 37.1 %, p = 0.303) of the 6d-SVBT group was not statistically different compared with that in the 6d-DVBT group. The multiple pregnancy rates (1.0 % in the 5d-SVBT group vs. 38.7 % in the 5d-DVBT group, p < 0.001 and 0 % in the 6d-SVBT group vs. 22.2 % in the 6d-DVBT group, p = 0.001) were statistically significantly lower in the SVBT group compared with those in the DVBT group regardless of the day of vitrification.
  
  
  CONCLUSIONS
  This study showed that the 5d-SVBT resulted in comparable clinical outcomes compared to the 5d-DVBT while the 6d-SVBT yielded significantly lower clinical outcomes compared to the 6d-DVBT.
authors.affiliation: Maria Fertility Clinic, 9th floor, Samsung Fire Bldg. 2-1 Beomeo 3-Dong, Suseong-Gu, Daegu, 706-743, Republic of Korea.
authors.forename: Sang Min
authors.initials: SM
authors.lastname: Kang
authors.forename: Sang Won
authors.initials: SW
authors.lastname: Lee
authors.forename: San Hyun
authors.initials: SH
authors.lastname: Yoon
authors.forename: Joo Cheol
authors.initials: JC
authors.lastname: Kim
authors.forename: Jin Ho
authors.initials: JH
authors.lastname: Lim
authors.forename: Seong Goo
authors.initials: SG
authors.lastname: Lee
mesh_terms: D000328:Adult; D001755:Blastocyst / Q000166:cytology*; D004624:Embryo Transfer / Q000379:methods*; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D011247:Pregnancy; D018873:Pregnancy Rate; D016896:Treatment Outcome; D058989:Vitrification