['To assess the effects of the nuclear status of day 2 preembryos on day 3 embryo quality and implantation potential and to weigh its clinical value in the human in-vitro fertilization-embryo transfer (IVF-ET) program.', 'Embryos obtained from 409 fresh conventional IVF-ET/ICSI cycles from July to October 2006 were assessed retrospectively. Day 2 preembryos were classified according to the number of nuclei in each blastomere in 3 groups: grade A with only mononucleated blastomeres, grade B with one or more blastomeres containing no visible nucleus, and grade C with one or more multinucleated blastomeres. Comparisons were made of the rates of arrested embryos and excellent embryos on day 3 as well as of the pregnancy outcome and implantation potential of those in whom cohorts of similar nuclear scoring embryos were transferred.', 'There were fewer arrested embryos and more excellent embryos on day 3 in grade A than in grade B and C (P < 0.01), and so were there in grade B than in grade C (P < 0.01). Among the 234 cycles in which all the transfer embryos were derived from a similar day 2 nuclear scoring, 51 cycles originated from grade A embryos (group A) and 183 from grade B (group B), with similar clinical pregnancy rates between the two groups, while the implantation rate was higher in group A than in B (P < 0.05).'] Does [ Nuclear status of day 2 preembryos affect embryo quality and implantation potential ]?