The rate of embryonic progression is more important to implantation and pregnancy rates than the embryonic stage of transfer: a comparison of day 5 and day 6 blastocyst transfers

Blastocyst culture and transfer is utilized as a tool to select embryos with higher implantation rates. Several studies have shown that inner cell mass (ICM) size and shape are important variables to predict blastocyst implantation potential. The objective of this study is to compare implantation rates following transfer of two blastocysts on day 5 or day 6 and to determine if culturing blastocyst embryos an additional 24 hours to day 6 of development will assist in the selection of blastocysts with the highest implantation potential. WIRB approved retrospective analysis of 2506 blastocyst embryo transfers from October 1999 through March 2007. The records of all patients having a blastocyst transfer were reviewed. Parameters selected for evaluation included day of blastocyst transfer, blastocoel expansion, and implantation rates. Blastocoel expansion was graded using Gardner's criteria (FS 1999) and categorized as morula, minimal (expansion 1–2), expanded (expansion 3–4) and hatched (expansion 5–6). Data were analyzed by complex Chi-square tests. Several embryological parameters were evaluated in relationship to the day of blastocyst transfer (Day 5 (n = 1791) vs. Day 6 (n = 715)) and the following findings were documented: a). The rate of blastocoel expansion irrespective of transfer day can be associated with higher clinical pregnancy rates [P<0.002]. b). The rate of blastocoel expansion irrespective of transfer day can be associated with improved implantation rates [P<0.002]. c). Day 5 blastocyst transfers had higher pregnancy and implantation rates than transfers performed on day 6 irrespective of blastocoel expansion [P<0.002]. These results suggest that the rate of embryonic progression as deemed by blastocoel expansion is an important indicator of embryo viability. Embryos that progress slowly to the blastocyst stage of development have significantly reduced implantation rates.