RESUMEN
PURPOSE: To relate pronuclear patterns (PN) and zygote cytoplasmic appearance and embryo morphology. METHODS: The usefulness of PN classification described by Tesarik et al. 1999 (patterns p0-5) and Scott et al. 2000 (Z1-4), for embryo selection is assessed. RESULTS: Sinchrony on polarization and number of nucleolar precursor bodies (NPB) were associated with good quality embryos (p0 60.9% and p3 67.3%, and Z1 62.5% and Z2 64.7%; p<0.01). Pattern 4 zygotes were associated with small number of NPB developed into multinucleated embryos (14.3%) and poor quality embryos (61.9%). No significant differences were found in the pregnancy rate between transfer of at least one good prognosis PN pattern and transfer of poor prognosis PN patterns, although 75% of the transfers included at least one embryo derived from a pattern 0 zygote, and 55% included embryos from categories Z1 or Z2. CONCLUSIONS: Sequential assessment involving the evaluation of oocyte quality, the classification of PN patterns and embryo morphology allows a more accurate evaluation of embryos to be selected for transfer.