2004) Developmental potential of human pronuclear zygotes in relation to their pronuclear orientation. Hum Reprod 19,294–299

BACKGROUND: Pronuclear zygote morphology has gained much attention recently due to its possible value in predicting implantation and pregnancy. The present study was performed to assess the developmental potential of zygotes with four different pronuclear orientations. METHODS: This prospective study involved 150 IVF and 190 ICSI patients seeking fertility treatment. Pronuclear zygotes were classi®ed for orientation of the pronuclei in rela-tion to the second polar body placed at the 6 o'clock position. Four types of pronuclear (PN) zygote were recognized, namely PN1, PN2, PN3 and PN4. The main outcome measures were early cleavage rate, quality of embryos, and implantation and pregnancy rates. RESULTS: The most common types of pronuclei orientations were PN1 (31.5%) and PN2 (29.3%), followed by PN3 (20.5%) and PN4 (18.5%). A signi®cantly higher proportion of zygotes with PN1 and PN4 types of pronuclear orientation underwent early cleavage and developed into grade I embryos compared with other types (P < 0.0001). There was a tendency for higher implantation and pregnancy rates among patients who received embryos developed from PN1- and PN4-type oocytes, but this was not statistically signi®cant. CONCLUSIONS: 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 PN1- and PN4-type zygotes, but implantation and pregnancy rates were not associated with pronuclear orienta-tion. Implantation and pregnancy rates tended to be higher for embryos developed from PN1 and PN4 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. Key words: early cleavage/polar body/pregnancy/pronuclear orientation/zygote polarit