title: Morphological scoring of human pronuclear zygotes for prediction of pregnancy outcome.
abstract: BACKGROUND
  As embryo selection is not allowed by law in Switzerland, we need a single early scoring system to identify zygotes with high implantation potential and to select zygotes for fresh transfer or cryopreservation. The underlying aim is to maximize the cumulated pregnancy rate while limiting the number of multiple pregnancies.
  
  
  METHODS
  In all, 613 fresh and 617 frozen-thawed zygotes were scored for proximity, orientation and centring of the pronuclei, cytoplasmic halo, and number and polarization of the nucleolar precursor bodies. From these individual scores, a cumulated pronuclear score (CPNS) was calculated. Correlation between CPNS and implantation was examined and compared between fresh and frozen-thawed zygotes. The effect of freezing on CPNS was also investigated.
  
  
  RESULTS
  CPNS was positively associated with embryo implantation in both fresh and frozen zygotes. With similar CPNS, frozen zygotes presented implantation rates as high as those of fresh zygotes. Nucleolar precursor bodies pattern and cytoplasmic halo appeared as the most important factors predictive of implantation for both types of zygotes, while pronuclei position was specifically relevant for frozen-thawed zygotes. Freezing induced an alteration of most zygote parameters, resulting in a significantly lower CPNS and a lower pregnancy rate.
  
  
  CONCLUSIONS
  CPNS may be used as a single prognostic tool for implantation of both fresh and frozen-thawed zygotes. Lower CPNS values of frozen-thawed zygotes may also be indicative of freezing damage to zygotes. Successful implantation of frozen zygotes despite lower CPNS suggests that they may recover after thawing and in vitro culture.
authors.affiliation: Reproductive Medicine Unit, Department of Gynecology and Obstetrics, CHUV, 1011 Lausanne, Switzerland. Alfred.senn@cpma.ch
authors.forename: Alfred
authors.initials: A
authors.lastname: Senn
authors.forename: Françoise
authors.initials: F
authors.lastname: Urner
authors.forename: Alain
authors.initials: A
authors.lastname: Chanson
authors.forename: Marie-Pierre
authors.initials: MP
authors.lastname: Primi
authors.forename: Daniel
authors.initials: D
authors.lastname: Wirthner
authors.forename: Marc
authors.initials: M
authors.lastname: Germond
mesh_terms: D002467:Cell Nucleus; D015925:Cryopreservation; D010064:Embryo Implantation; D005260:Female; D006801:Humans; D011247:Pregnancy; D011256:Pregnancy Outcome; D011379:Prognosis; D015053:Zygote / Q000166:cytology*