Developmental potential of human pronuclear zygotes in relation to their pronuclear orientation

Zygotes exhibit four types of pronuclear orientation, and this is independent of the fertilizing spermatozoon or its entry point into the oocytes, whether IVF or ICSI is employed. Early cleavage was associated with PN(1)- and PN(4)-type zygotes, but implantation and pregnancy rates were not associated with pronuclear orientation. Implantation and pregnancy rates tended to be higher for embryos developed from PN(1) and PN(4) pronuclear zygotes. Further studies on a combination of pronuclear orientation and equality of nucleoli may provide a better guide to the implantation potential of embryos.