Parameters guiding selection of best embryos for transfer after cryopreservation: a reappraisal.

Background: The purpose of this study was to evaluate the respective influences of blastomere survival and resumption of mitosis on the outcome of frozen-thawed embryos.

Methods: A retrospective analysis was performed in our centre on 363 thawing cycles, involving 4-cell day 2 grade 1 embryos with <10% fragmentation.

Results: A higher implantation rate per transferred embryo was observed when all transferred embryos were characterized by fully intact blastomeres (100% blastomere survival) as compared with damaged embryos (50 or 75% blastomere survival) (22.0 versus 7.2%; P < 0.0001). Moreover, the implantation rate per transferred embryo was significantly higher for cleaved embryos compared with uncleaved embryos (19.7 versus 3%; P < 0.0001). Transfer of fully intact, cleaved embryos resulted in the highest implantation rates compared with transfer of damaged and uncleaved embryos (27.4 versus 0%; P < 0.0001). Intermediate implantation rates were observed when only one of the two criteria was fulfilled (13 versus 11% respectively; P > 0.05). Multivariate analysis showed that the clinical pregnancy rate was influenced by both criteria (odds ratio = 3.4 for transfer of embryos with six or more cells versus embryos with less than six cells.

Conclusion: The results of our study suggest that the most important factor to predict further embryo development is the total number of blastomeres in transferred embryos, however they are obtained (good survival and/or resumption of mitosis).