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 and blastocyst stages. Zygote morphology has been related to implantation and pregnancy. The aim of this study was to relate pronuclear morphology to blastocyst development. Zygotes were scored according to distribution and size 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 morphology on day 3 had increased implantation (IR) and pregnancy rates (PR; 31 and 57%), compared with those selected by morphology alone (19 and 33% respectively; P: < 0.01). There was a significant difference between zygote-scored 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 maintain pregnancy rates for both day 3 and day 5 transfers, increase implantation rates and reduce the numbers of embryos required to achieve a pregnancy.