How viable are zygotes in which the PN are still intact at 25 hours? Impact on the choice of embryo for transfer

Objective: To evaluate the pertinence of kinetic observations of embryonic development 25 hours after IVF, with a view to optimizing the choice of embryos for transfer. Design: Retrospective study of 471 cycles with embryo transfer. Setting: University Hospital. Intervention(s): Observations of changes in zygotes, in the 25th hour after insemination (25 h PI). Main Outcome Measure(s): Identification of three stages of development: two cells (2C), loss of the pronuclei (in syngamy; 0PN) and persistence of the two pronuclei (2PN). Embryonic morphology at 48 hours, according to the stages identified at 25 hours, was correlated with the rates of implantation and clinical pregnancy. Result(s): Two thousand four hundred seventy-eight day-2 embryos were observed. The number of blastomeres on day 2 differed significantly according to stage of development at 25 hours (2C: 69.8% vs. 0PN: 55% and 2PN: 24.6%). Implantation rates were significantly lower for cycles in which only 2PN embryos were produced (7%, n = 160) than for cycles with at least one 2C embryo and/or, preferably, a 0PN embryo (14.4%). The implantation rate was significantly lower for the transfer of only 2PN embryos (7.6%) than for the transfer of only embryos that had advanced further by 25 h PI (15.2%). If only one type of embryo was transferred (2PN vs. 2C or 0PN), for embryos with a similar morphology at 48 hours, four-blastomere embryos derived from 2PN embryos implanted less efficiently than those derived from 2C and/or 0PN embryos. Conclusion(s): Observation at 25 h PI is a simple, rapid, and cheap method for identifying the best embryos to transfer. A lack of zygote development at 25 hours is associated with a poor prognosis, unless the zygote concerned has two to three cells on day 2. The development of zygotes with 2PN at 25 h PI should be taken into account as a function of their development at 48 hours.