The morphology of human pronuclear embryos is positively related to blastocyst development and implantation

Human embryos are selected for transfer using morphology at the cleaving an d blastocyst stages. Zygote morphology has been related to implantation and pregnancy. The aim of this study was to relate pronuclear morphology to bl astocyst development. Zygotes were scored according to distribution and siz e of nucleoli within each nucleus. Zygotes displaying equality between the nuclei had 49.5% blastocyst formation and those with unequal sizes, numbers or distribution of nucleoli had 28% blastocyst formation. Cleaving embryos that were selected initially by zygote morphology and secondarily by morph ology on day 3 had increased inplantation CLR and pregnancy rates (PR; 31 a nd 57%), compared with those selected by morphology alone (19 and 33% respe ctively; P < 0.01). There was a significant difference between zygote-score d and non-scored cycles on day 3 (PR: 57 versus 33%; IR: 31 versus 19%) and on day 5 (PR: 73 versus 58%; IR; 52 versus 39%). Zygote scoring can mainta in pregnancy rates for both day 3 and day 5 transfers, increase implantatio n rates and reduce the numbers of embryos required to achieve a pregnancy.