Using rigorously matched non-frozen controls we have shown that cryopreserv
ation does not alter the implantation potential of early cleavage stage (da
y 2) human embryos if no blastomere loss occurs. Thawed intact 4-cell embry
os have a significantly higher implantation (fetal heart) rate (16.9%) than
similar 2-cell embryos (7.2%). This difference is not due to blastomere nu
mber per se since increasing the cell number in frozen embryos by allowing
an extended period in culture prior to freezing does not alter their intrin
sic 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 preimpla
ntation and peri-implantation losses may contribute to this outcome. (C) 20
00 Elsevier Science Ireland Ltd. All rights reserved.