The limited importance of pronuclear scoring of human zygotes.

Background: Several studies have shown a correlation between the pronuclear morphology score (PNMS) and subsequent embryo development and implantation. Embryos with poor pronuclear score, elsewhere referred to as Z3 and Z4, are often not transferred or cryopreserved because it is believed that they have poor pregnancy potential. The objective of this study is to report our data on the use of the pronuclear score and its effect on pregnancy outcome.

Methods: Retrospective analysis of IVF/ICSI-embryo transfer cycles completed over the course of 1 year (n = 334). Comparisons were made only in those groups of patients in whom cohorts of similarly scored PNMS embryos were transferred. The proportion of such homologous cohorts was 104/334 (31%). All other replacements were excluded from final analysis as they were dissimilar as far as PNMS is concerned. Pregnancy outcomes were evaluated.

Results: The incidence of live birth resulting from the transfer of single pronuclear score homologous embryo types was 56 (14/25), 41 (13/32), 54 (23/43) and 0% (0/4) for PNMS scores 1, 2, 3 and 4, respectively. There was no correlation between PNMS category of the embryos transferred and live birth rates (P = 0.139).

Conclusions: PNMSs of 1, 2 or 3 do not correlate with live birth rates when assessing unique PNMS embryo transfers. In particular, previously considered poor (type 3) embryos can result in pregnancy with normal live birth rates. Whether type 4 embryos are compatible with normal development remains to be shown.