A quantitative analysis of the impact of cryopreservation on the implantation potential of human early cleavage stage embryos

The impact of cryopreservation on the implantation potential of early cleav age stage (day 2) embryos was assessed by analysing the outcome from > 5000 thawed embryos in relation to the outcome from a similar number of fresh e mbryos. Analysis of procedures in which all transferred embryos fulfilled e quivalent defined criteria revealed no significant difference in the implan tation rates (fetal hearts/ 100 embryos transferred) of fresh 4-cell embryo s (16.6%) and fully intact thawed 4-cell embryos (16.9%). Although 2-cell e mbryos implanted at significantly lower rates, there was again no significa nt difference between fresh (6.5%) and fully intact thawed (7.2%) embryos. Similar analysis of all embryos (irrespective of cell number on day 2) demo nstrated that the implantation potential of partially intact thawed embryos was related to the extent of blastomere loss with the implantation rate of embryos with 50% cell survival (5.4%) being approximately half the rate of fully intact embryos (11.3%). Combining the values obtained from 'pure' da ta for the implantation rates of embryos with defined levels of survival wi th their relative prevalence in the total population of thawed embryos gave a predicted number of implantations (441) which was similar to the observe d outcome (463). This number was similar to 30% less than the number expect ed had the same embryos been transferred fresh (635). The results suggest t hat intact thawed embryos have the same implantation potential as equivalen t fresh embryos and that the impact of cryopreservation is limited to blast omere loss which is directly related to loss of implantation potential. The observed frequency of blastomere loss results in a reduction of similar to 30% in the implantation potential of a population of embryos following cry opreservation.