The pronuclei in human one-cell stage embryos (zygotes) contain several nucleoli called nucleolus precursor bodies (NPBs). Based on their number and distribution, it is possible to predict the developmental potential of the zygote. Recently, it has been demonstrated that the speed of NPBs movement in pronuclei may also indicate how the embryo will develop, as well as its chromosomal constitution (euploidy versus aneuploidy). These observations, however, do not elucidate the mechanisms behind these processes, and a deeper understanding will certainly be important for the more efficient production of healthy human embryos.