Pronuclear scoring. Time for international standardization.

Zygote scoring is an efficient tool for embryo selection not only in countries where embryo selection is not permitted. Several different scoring systems have been published so far, making comparisons of assessments between investigators and laboratories extremely difficult. Pronuclear evaluation should be standardized in a manner analogous to the standardization of cleavage stage embryo scoring or of semen evaluation by the World Health Organization. The ideal score should be clear and easily applicable. The items that have the greatest influence on embryonic development seem to be alignment and size of pronuclei, alignment and number of nucleoli, halo effect and appearance of vacuoles. These morphologic parameters can be observed in different features and can be summarized as a zygote score.