Abstract
Despite advancements in technologies such as time-lapse microscopy and artificial intelligence, the gold standard for embryo selection still relies on standard morphological assessment. Several studies have investigated the correlation between blastocyst characteristics (expansion status, inner cell mass, and trophectoderm) and clinical outcomes, reaching contradictory results. In consideration of these ambiguities in the literature, we performed a retrospective study of 1546 untested first-vitrified-warmed single day 5/6 blastocyst transfers. The purpose of our study is to evaluate three scenarios: (i) independent association between each morphological characteristic (expansion status, inner cell mass, and trophectoderm) and live birth; (ii) comparison between blastocysts with inner cell mass grade A and trophectoderm grade B and blastocysts with inner cell mass grade B and trophectoderm grade A; and (iii) comparison between poor-quality day 5 and top-quality day 6 blastocysts. After adjusting for principal confounders, we report that trophectoderm is more predictive of live births than inner cell mass and expansion status. We observed a trend in favor of top-quality day 6 blastocysts over poor-quality day 5 blastocysts. Moreover, on the same day of development and expansion status, blastocyst BA should be preferable to blastocyst AB.