The developmental potential of cryopreserved human embryos.

Using rigorously matched non-frozen controls we have shown that cryopreservation does not alter the implantation potential of early cleavage stage (day 2) human embryos if no blastomere loss occurs. Thawed intact 4-cell embryos have a significantly higher implantation (fetal heart) rate (16.9%) than similar 2-cell embryos (7.2%). This difference is not due to blastomere number per se since increasing the cell number in frozen embryos by allowing an extended period in culture prior to freezing does not alter their intrinsic developmental potential. Blastomere loss, which occurred in almost half of all thawed embryos, is directly related to a reduction in developmental potential. We estimate that approximately 30% of the expected fresh embryo implantations are lost as a consequence of cryopreservation. Both preimplantation and peri-implantation losses may contribute to this outcome.