title: The freezing method of cleavage stage embryos has no impact on the weight of the newborns.
abstract: OBJECTIVE
  The aim of this study was to study the effect of the embryo freezing method on the birth weight of newborns from frozen embryo transfer (FET) cycles, and the pregnancy results of cleavage stage embryos cryopreserved by slow freezing or vitrification.
  
  
  METHODS
  This is a retrospective cohort study undertaken in a University Hospital IVF unit using concurrently both the slow-freezing and the vitrification techniques. All frozen-thawed and vitrified-warmed day 2 and day 3 embryo transfers during the time period from 1 April 2009 to 31 November 2013 were included in the study.
  
  
  RESULTS
  There was no statistically significant weight difference between newborns from vitrified or slow-frozen embryos (3588 vs 3670 g). A higher post-thaw viability rate was achieved after cryopreservation by the vitrification technique compared to the slow-freezing protocol (83.4 vs 61.4%). The miscarriage rate was lower in the vitrification group (15.7 vs 29.0%). The live birth rates were similar (19.5 vs 19.1%) in the slow-freezing and vitrification groups, respectively. Among vitrified embryos, 7.4 embryos needed to be thawed to produce one delivery; in the slow-freezing group, that number was 11.9.
  
  
  CONCLUSIONS
  The freezing method has no impact on the weight of the newborn. With lower post-thaw survival rates and higher miscarriage rates, the slow-freezing cryopreservation protocol is inferior to the vitrification technique.
authors.affiliation: Department of Obstetrics and Gynaecology, Tampere University Hospital, University of Tampere, Teiskontie 35, 33521, Tampere, Finland. noora.kaartinen@pshp.fi.
authors.forename: N
authors.initials: N
authors.lastname: Kaartinen
authors.affiliation: Infertility Clinic Ovumia, Biokatu 12, 33520, Tampere, Finland.
authors.forename: K
authors.initials: K
authors.lastname: Kananen
authors.affiliation: School of Health Sciences, Tampere University, T-Building, Medisiinarinkatu 3, 33520, Tampere, Finland.
authors.forename: H
authors.initials: H
authors.lastname: Huhtala
authors.affiliation: Central Finland Central Hospital, Keskussairaalantie 19, 40620, Jyväskylä, Finland.
authors.forename: S
authors.initials: S
authors.lastname: Keränen
authors.affiliation: Department of Obstetrics and Gynaecology, Tampere University Hospital, University of Tampere, Teiskontie 35, 33521, Tampere, Finland.
authors.forename: H
authors.initials: H
authors.lastname: Tinkanen
mesh_terms: D000328:Adult; D001723:Birth Rate; D001724:Birth Weight / Q000502:physiology*; D001755:Blastocyst / Q000166:cytology / Q000502:physiology*; D015331:Cohort Studies; D015925:Cryopreservation / Q000379:methods*; D004624:Embryo Transfer / Q000379:methods*; D005260:Female; D005307:Fertilization in Vitro / Q000379:methods; D006801:Humans; D007231:Infant, Newborn; D008297:Male; D011247:Pregnancy; D018873:Pregnancy Rate; D012189:Retrospective Studies; D058989:Vitrification
keywords: Cryopreservation; Frozen embryo transfer; Slow freezing; Vitrification