Resumption of mitosis during post-thaw culture: a key parameter in selecting the right embryos for transfer.

This retrospective study of 701 thaw cycles analysed the clinical importance of whether or not embryos resumed mitosis during 24 h of post-thaw culture. A total of 3360 frozen embryos were thawed; 1922 embryos survived the freeze-thaw procedure with at least one intact blastomere and were then cultured for 24 h before transfer. All transfers were registered into either the 'cleaved embryo group' (n = 459), which was defined as transfers where at least one of the transferred embryos cleaved during the post-thaw culture period, or the 'non-cleaved embryo group' (n = 153), where none of the transferred embryos cleaved during the post-thaw culture period. A total of 1408 thawed embryos were transferred in 612 cycles; 459 embryo transfers were in the cleaved embryo group, resulting in an implantation rate of 10%, significantly higher than the 4% in the non-cleaved embryo group (P = 0.0003). A total of 130 pregnancies (28% per transfer) were obtained in the cleaved embryo group which was significantly higher than the 17 pregnancies (11% per transfer) obtained in the non-cleaved embryo group (P = 0.0001). However, the average number of transferred embryos was significantly higher in the cleaved embryo group (2.46 +/- 0.03) compared to the non-cleaved embryo group (1.82 +/- 0.07). No difference was found in the age of the women between the two groups. When analysing transfers where all transferred embryos had cleaved during the post-thaw culture period the clinical pregnancy rate increased significantly from 13% transferring two embryos to 36% transferring three embryos (P = 0.0136). In this latter subgroup an implantation rate as high as 17% was obtained. The overall multiple pregnancy rate was 16%. The multiple pregnancy rate was 19% in the cleaved embryo group. In conclusion, 24 h post-thaw culture may allow a better selection of the embryos and thereby we may be able to increase the implantation and pregnancy rates. This may enable us further to reduce the number of embryos transferred.