Incidence and development of zygotes exhibiting abnormal pronuclear disposition after identification of two pronuclei at the fertilization check

Objective: To determine the incidence, developmental potential, and clinical implications of embryos having one pronucleus (1PN) or three pronuclei (3PN) at early cleavage, despite exhibiting 2PN at the fertilization check. Design: Retrospective cohort study. Setting: Hospital-based academic medical center. Patient(s): All IVF cycles from January 2006 through May 2008 having 2PN zygotes that subsequently transitioned to 1PN or 3PN before cleavage, matched to cycles having 2PN zygotes progressing to cleavage without intervening abnormal pronuclear disposition. Intervention(s): Standard IVF protocol. Main Outcome Measure(s): Incidence, day 3 development, and implantation rates of 2PN zygotes transitioning to 1PN and 3PN states before cleavage, compared with normal embryos. Result(s): The incidences of 1PN and 3PN zygotes were 2.9% and 0.4%, respectively. Both types of abnormal zygote showed slower day 3 cleavage, although only the 1PNs exhibited higher fragmentation and asymmetry compared with controls. The 1PN zygotes had a 6.4% implantation rate and viable pregnancy rate of 1.3%. Of the nine 3PN zygotes transferred, none implanted. Conclusion(s): Two-pronuclear zygotes transitioning through 1PN or 3PN states tend to develop into poorer-quality embryos than 2PN control zygotes. Patients should be counseled regarding the very low likelihood of viable pregnancy after transfer of these abnormally developing zygotes. (Fertil Steril (R) 2010; 94: 965-70. (c) 2010 by American Society for Reproductive Medicine.)