Day 6 blastocyst is associated with increased birth weight in full-term singleton newborns after frozen-thawed transfer. 
The purpose of the study is to compare the newborns weight in singleton term birth following transfer of thawed blastocysts-frozen on either day 5 or day 6 after in vitro fertilization.  The retrospective study included 1444 frozen-thawed blastocyst transfer (FBT) cycles resulting in live singleton births between Jan 2013 and Dec 2016.  The main outcomes measured were absolute birth weight, z-score adjusted for gestational age and gender, and incidence of large-for-gestational-age (LGA) newborns.  Generalized linear model (GLM) and logistic regression were used in multivariate analyses.  Both the absolute birth weight (3416.49 ± 404.74 vs 3349.22 ± 416.17) and the z-score (0.6 ± 0.93 vs 0.41 ± 0.93) were significantly higher on day 6 FBT in comparison with day 5 FBT.  The incidence of LGA newborns was also increased on day 6 FBT (22.8 vs 14.7%, P = 0.006).  Adjusted for maternal age, BMI, PCOS diagnosis, present of vanishing twin, and embryo quality, the odds ratio (95% confidence interval) for LGA on day 6 FBT comparing with day 5 FBT was 1.76 (1.18-2.64).  Day 6 FBT is associated with increased birth weight and contributes to the incidence of LGA newborns in FBT.