The Effect of the Day 3 Embryo Cell Number on the Neonatal Outcomes of Day 5 Single Blastocyst Transfer in Frozen Embryo Transfer Cycles

Objective: To evaluate the influence of the day 3 embryo cell number on the neonatal outcomes of day 5 single blastocyst transfer in frozen embryo transfer (FET) cycles.Methods: This retrospective study analysed a total of 2315 delivery cycles of day 5 single blastocyst transfer in FET cycles, including 489, 761 and 1103 live-born infants segregated according to a day 3 embryo cell number of <8, 8 and >8 cells, respectively. The neonatal outcomes of the three groups were compared.Results: The day 3 embryo cell number did not significantly affect the incidence of monozygotic twins. The sex ratio increased as the day 3 embryo cell number increased, but the difference was not statistically significant. There were no significant differences in the rates of preterm birth or low birth weight among the three groups. The rates of stillbirths and neonatal deaths were also not significantly different among the three groups. Moreover, the day 3 embryo cell number did not increase the risk of birth defects in newborns.Conclusions: The day 3 embryo cell number did not significantly affect neonatal outcomes.