title: Effect of developmental stage of embryo at freezing on pregnancy outcome of frozen-thawed embryo transfer.
abstract: BACKGROUND
  The study aim was to investigate the impact of the developmental stage of embryos on pregnancy outcome of frozen embryo transfer (FET).
  
  
  METHODS
  The survival rates of embryos after thawing and pregnancy outcome following FET were compared retrospectively between three cryopreservation strategies utilizing either zygote, day 2 or day 3 embryo freezing.
  
  
  RESULTS
  A total of 4006 embryos was analysed in 1657 thaw cycles. The highest (P < 0.0001) survival rate (all cells survived) was observed for zygotes (86.5%), followed by day 2 (61.7%) and day 3 (43.1%) embryos. FET was performed in 1586 (95.7%) of all thaw cycles, resulting in overall clinical pregnancy and implantation rates of 20.7 and 14.2% respectively. The delivery rate per transfer was 16.5%, and live birth rate per transferred embryo 11%. There were no significant differences in clinical pregnancy, implantation, delivery and birth rates between frozen zygote, day 2 and 3 embryo transfers. However, an elevated miscarriage rate was observed in the day 3 group (45%) compared with zygotes (21.3%; P = 0.049) and day 2 embryos (18.3%; P = 0.004). The overall efficacy of FET (birth rate per thawed embryo) was 7.3%. The efficacy was lower in day 3 group (4.2%) than in the zygote (7.1%; P = 0.082) and day 2 (7.6%; P = 0.027) groups.
  
  
  CONCLUSIONS
  The developmental stage of embryos at freezing has a profound effect on their post-thaw survival, but seems to have little effect on rates of clinical pregnancy, implantation, delivery and birth after FET. The elevated miscarriage rate for day 3 frozen embryo transfers may be caused by damage during freeze-thaw procedures. The low survival rate and elevated miscarriage rate were both responsible for a reduced overall efficacy for day 3 FET when compared with zygotes and day 2 embryos.
authors.affiliation: Infertility Clinic, The Family Federation of Finland, Kalevankatu 16A, FIN-00100 Helsinki, Finland. andres.salumets@vaestoliitto.fi
authors.forename: Andres
authors.initials: A
authors.lastname: Salumets
authors.forename: Timo
authors.initials: T
authors.lastname: Tuuri
authors.forename: Sirpa
authors.initials: S
authors.lastname: Mäkinen
authors.forename: Sirpa
authors.initials: S
authors.lastname: Vilska
authors.forename: Lea
authors.initials: L
authors.lastname: Husu
authors.forename: Ritva
authors.initials: R
authors.lastname: Tainio
authors.forename: Anne-Maria
authors.initials: AM
authors.lastname: Suikkari
mesh_terms: D000022:Abortion, Spontaneous / Q000453:epidemiology; D001723:Birth Rate; D015925:Cryopreservation; D036861:Delivery, Obstetric / Q000706:statistics & numerical data; D010064:Embryo Implantation; D004624:Embryo Transfer; D005314:Embryonic and Fetal Development; D005260:Female; D006801:Humans; D011247:Pregnancy; D011256:Pregnancy Outcome; D018873:Pregnancy Rate; D012189:Retrospective Studies