title: High pregnancy rates can be achieved after freezing and thawing human blastocysts.
abstract: OBJECTIVE
  To examine the results of a 3-year trial using blastocyst cryopreservation to limit multiple pregnancy and optimize overall pregnancy per cycle.
  
  
  METHODS
  Retrospective clinical evaluation of pregnancy rates after freezing and thawing human blastocysts.
  
  
  METHODS
  Tertiary-care academic center.
  
  
  METHODS
  Seven hundred fifty-three different patients treated in 783 IVF cycles with blastocysts frozen from July 2000 to June 2003.
  
  
  METHODS
  Two thousand, two hundred fifty-nine blastocysts were frozen in cycles in which only blastocysts were cryopreserved (cycles with pronuclear stage oocytes or pre-embryos also cryopreserved were excluded from the analysis). Of these, 628 (27.6%) were thawed in 218 cycles.
  
  
  METHODS
  Pregnancy rate per cycle with thaw.
  
  
  RESULTS
  Four hundred seventy-nine (76.3%) blastocysts survived thawing, and 440 (92.0%) were transferred after exhibiting evidence of survival (most commonly, blastocoele reexpansion). In cycles with a thaw, 211 (96.8%) of 218 underwent intrauterine transfer. An average of 2.09 blastocysts was transferred per replacement. One hundred twenty-five (59.2%) clinical pregnancies were established, which included 23 sets of twins and 5 triplet gestations. Two sets of monozygotic twins were identified after the replacement of a single thawed blastocyst (1.6%). The age of the patient at the time of cryopreservation (<37 years) was an important factor in the establishment of clinical and ongoing pregnancy. The mode of ovarian stimulation, replacement method, and whether blastocysts were frozen on day 5 or day 6 of development did not demonstrate clinical significance.
  
  
  CONCLUSIONS
  Cryopreserved and thawed blastocysts demonstrated a similar potential for implantation when compared with fresh pre-embryos on day 3. On the basis of these results, the blastocyst stage of development appears to be optimal for clinical freeze-thaw trials.
authors.affiliation: The Center for Reproductive Medicine and Infertility, Weill-Cornell Medical College, New York, New York 10021, USA. lveeck@med.cornell.edu
authors.forename: Lucinda L
authors.initials: LL
authors.lastname: Veeck
authors.forename: Richard
authors.initials: R
authors.lastname: Bodine
authors.forename: Robert N
authors.initials: RN
authors.lastname: Clarke
authors.forename: Rosemary
authors.initials: R
authors.lastname: Berrios
authors.forename: Joanne
authors.initials: J
authors.lastname: Libraro
authors.forename: Rose Marie
authors.initials: RM
authors.lastname: Moschini
authors.forename: Nikica
authors.initials: N
authors.lastname: Zaninovic
authors.forename: Zev
authors.initials: Z
authors.lastname: Rosenwaks
mesh_terms: D000328:Adult; D001755:Blastocyst; D015925:Cryopreservation; D004624:Embryo Transfer; D005260:Female; D005307:Fertilization in Vitro; D006801:Humans; D008423:Maternal Age; D011247:Pregnancy; D018873:Pregnancy Rate; D011272:Pregnancy, Multiple; D012189:Retrospective Studies; D014310:Triplets; D014427:Twins; D014430:Twins, Monozygotic