Pronuclear scoring as a predictor of embryo development.

Many strategies have been proposed for the selection of viable embryos for transfer in human assisted reproduction. These have included morphological scoring criteria for day 1, 2, 3 and 5 embryos or combinations of these. Other strategies have used predictors such as timing of certain key events, as with early cleavage to the 2-cell, development to the 8-cell stage or patterns of fragmentation. All have shown some correlations with implantation. However, the overall success of these methods is still limited, with over 50% of all transferred embryos failing to implant. The use of pronuclear oocyte morphology has shown correlations with implantation and development to the blastocyst stage. The key aspects of pronuclear scoring, namely the presence of a cytoplasmic halo, the orientation of the nuclei in relation to the polar bodies and the size, number and pattern of distribution of nucleolar precursor bodies (NPB) in the nuclei were related to day 2,3 and 5 development, rate of development and day 3 and 5 morphology in a retrospective study. The pattern of the NPB or Z-score and the presence/absence of a halo had a significant effect on the rate of development on day 3 and day 5 and on the overall embryo morphology score. Low Z-score resulted in slow development, poor blastocyst formation and low morphology scores. The absence of a halo also resulted in slow and poor development, poor morphology, increased fragmentation and increased numbers of poor Z-scored embryos. The use of PN scoring can help predict embryos that have poor developmental potential, aid in early selection and may indicate the health of the oocyte.