Embryo selection or uterine environment: which plays the greater role in blastocyst transfer cycles?

Blastocyst stage embryo transfer (BET) leads to higher pregnancy rates when compared to cleavage stage transfer. Better embryo selection and a more physiologic stage of transfer are possible explanations. We assessed the significance of embryo selection in achieving an improved outcome.

Retrospective analysis of all BETs from our 2007 IVF database was performed. The BET cycles were grouped based on the number of top-quality cleavage stage embryos (≤3 or >3). A cut-off of ≤3 embryos was chosen, because with ≤3 embryos those that will likely make it to the blastocyst stage have already been identified, therefore embryo selection becomes less important. Baseline characteristics, stimulation and embryology parameters, as well as treatment outcome, were compared. The T-test and χ(2)-test were used where appropriate. P < 0.05 was considered significant.

A total of 251 cycles where at least one blastocyst was transferred on day 5 were identified and included in the analysis (≤3 top-quality cleavage embryos, n = 32; >3 top-quality cleavage embryos, n = 219). The number of follicles >14 mm and the number of eggs/mature eggs were higher in the group with >3 good quality day 3 embryos. Slightly more embryos were transferred in the ≤3 embryo group. Pregnancy rates (14/32 [43.8%] vs 96/219 [43.8%]; P = NS) and ongoing pregnancy rates (10/32 [31.2%] vs 80/219 [36.5%]; P = NS) were comparable.

Based on the analysis of our data, the number of top-quality cleavage stage embryos did not significantly influence BET outcome. Besides embryo selection, other factors such as improved embryo-endometrium synchrony probably play an important role in the higher implantation and pregnancy rates.