Background: Embryo quality may be accurately assessed as early as the pronuclear zygote phase, as shown in recent studies. However, it is not known whether good quality zygotes are destined to become good quality cleavage stage embryos and blastocysts.
Methods: In this retrospective study, 86 intracytoplasmic sperm injection-embryo transfer cycles were studied where each available embryo was scored from the zygote until the blastocyst stage. Embryonic normality parameters such as pronuclear pattern, early cleavage, cleavage stage embryo grade, the presence of embryos with > or =8 cells on day 3 and blastocyst quality were recorded. Embryo transfer was undertaken at the blastocyst stage and the outcome was studied according to the pronuclear pattern exhibited by the zygotes.
Results: Embryos that showed an ideal pronuclear pattern (0 PN pattern) cleaved earlier and faster and resulted in better quality cleavage stage embryos and blastocysts. The incidence of blastocyst formation was 72% in zygotes showing a 0 PN pattern, compared with 12.7% in zygotes with double pronuclear abnormality. Higher implantation and pregnancy rates were obtained when at least one blastocyst derived from a 0 PN pattern zygote was included in the set of embryos to be transferred.
Conclusions: Our results indicate that the pronuclear pattern of the zygote is closely related to blastocyst formation and quality. Blastocysts derived from 0 PN zygotes have a higher potential for implantation.