Slow Development of a Day 5 Fresh Transferred Embryo Leads to Reduced Live Birth Rate but Not Birthweight: A Multicenter Retrospective Cohort Study

Objective: This study aims to investigate the impact of slow embryo development on the subsequent live birth rate and birthweight outcomes following a fresh day 5 transfer. Study design: This retrospective multicenter cohort study included 1,213 consecutive patients undergoing autologous oocyte in vitro fertilization (IVF) treatment at four associated private clinics during 2016–2019. Only fresh single day 5 transfers were included for analysis. Results: No implantation was achieved by embryos that failed to reach the early blastocyst stage on day 5 ([Formula: see text]). After adjusting for potential confounding factors, multivariate logistic regression (expressed as adjusted odds ratio or aOR and [Formula: see text] confidence interval) showed a significantly reduced live birth rate in early blastocysts ([Formula: see text]) in reference to those at the expanding (aOR = 0.584, [Formula: see text], [Formula: see text], [Formula: see text]), expanded (aOR = 0.322, [Formula: see text], [Formula: see text], [Formula: see text]), or hatching/hatched stages (aOR = 0.255, [Formula: see text], [Formula: see text], [Formula: see text]). However, early blastocysts led to similar birthweights ([Formula: see text], [Formula: see text]) in comparison to those at the expanding ([Formula: see text], [Formula: see text], [Formula: see text]), expanded ([Formula: see text], [Formula: see text], [Formula: see text]), or hatching/hatched stages ([Formula: see text], [Formula: see text], [Formula: see text]). This was further confirmed by linear regression analysis using either birthweight or [Formula: see text] score (gestation-adjusted birthweight). Conclusion: Slow day 5 development is associated with reduced live birth rate when transferred fresh, however, subsequent birthweight is not impacted once pregnancy is initiated.