Is Blastocyst Culture Responsible for Higher Pregnancy Rates? A Critical Analysis of the Day of Optimal Embryo Transfer and Embryo Quality

Objective: A prolonged culture of embryos beyond day 2-3 to day 5 (blastocyst culture) after fertilization might be an alternative, simple way of selecting suitable embryos for transfer. Extending embryo culture to day 5/6 is a selection tool to choose an embryo with a greater likelihood of implantation rather than improve embryo quality. Methods: This retrospective study analyzed 1126 fresh IVF/ICSI cycles performed between February 1, 2014 and December 30, 2018 at the University Fertility Center in Kiel, Germany, to determine the impact of blastocyst culture on pregnancy rates and the association between embryo quality and pregnancy rates. Results: Clinical pregnancy was achieved in 154 cases (19.5%) after day 2/3 transfer and in 76 cases (22.7%) after day 5 transfer. Pearson's two-sided chi-squared test yielded no statistical significance (p=0.221). The analysis of clinical pregnancy rates in relation to the quality of transferred embryos yielded the following results: 49 (10.7%) pregnancies in cases of no ideal embryo(s); 122 (27.2%) in cases of at least one ideal embryo; and 59 (26.7%) for both quality groups. Pearson's two-sided Chi-squared test was statistically significant (p<0.001). Conclusions: Our data revealed no improvement of pregnancy rates after blastocyst transfer compared with day 2/3 transfers. However, we noted higher pregnancy rates when an embryo of good quality was transferred.