Objective       To study the correlation of pronucleus scoring system with embryo morphology 3 days after fertilization

blastulation and implantation rate 6 days after fertilization in order to assess its application in in vitro fertilization-embryo transfer (IVF-ET).      Methods       Fertilization was observed 16-20 h after insemination during conventional IVF or intracytoplasmic sperm injection (ICSI). Normal fertilized zygotes were divided into groups Z1

Z2

Z3 and Z4 according to the Z scoring system developed by Scott. Cleavage rate and excellent embryo rates 3 days after fertilization and blastulation 6 days after fertilization were compared between different pronuclei. Implantation rates of different fertilized eggs were compared by analyzing the transferred embryos.      Results       The cleavage rate (92.9%)

excellent embryo rate 3 days after fertilization (14.25%) and blastulation rate 6 days after fertilization (10.48%) were significantly lower in Z4 group than in the other 3 groups (P0.001). The implantation rate was very low in Z4 group（5.88%）with no significant difference among the 4 groups（P0.05）. Significant difference was also not observed in excellent embryo and blastulation and implantation rates among the Z1

Z2 and Z3 groups（P0.05）.      Conclusion       Abnormal pronucleus morphology (Z4) can predict the in vitro and in vivo developmental potential of embryos

but the morphology of nucleoli does not necessarily predict the embryo viability. Z1

Z2 and Z3 embryos may have the same development potential.