Cleavage vs blastocyst stage embryos: how are they interrelating?

Purpose: To assess the variables that may predict which cleavage-stage embryo may develop into a blastocyst, and vice versa, to determine whether the cleavage-stage embryo morphology should be taken into consideration when transferring the embryo at the blastocyst stage. Methods: A single center, retrospective cohort study. The study cohort included 3072 patients undergoing 3607 retrieval cycles and 23,124 embryos at the cleavage stage. We assessed the blastulation rate and evaluated which variables impact the ongoing pregnancy rate. Results: High blastulation rate correlates with higher embryos' grading (I > II > III > IV > V) and higher number of blastomeres (8 > 7 > 6 > 5 > 4). 949 patients had fresh single blastocyst transfers. The ongoing pregnancy rate was 28.9% per transfer. Patients with ongoing pregnancies were significantly younger (34.3 vs. 36 years, p < 0.001), had higher number of oocyte yield (9.8 vs. 9, p = 0.02), and an increased rate of good-quality embryos transferred (70.7% vs. 47.7%, p = 0.001). When evaluating embryos progression, we found that whenever embryo developed to a good-quality blastocyst, its appearance at the cleavage stage did not affect ongoing pregnancy rate. Conclusion: Higher the number of blastomeres and better embryo grading were found to correlate with a higher blastulation rate. Nevertheless, if the embryo has already developed to a top-quality blastocyst, its morphology at the cleavage stage did not impact ongoing pregnancy rate.