2-3 and Day 5 fresh and frozen-thawed embryos are most frequently used in ART, while the results of Day 4 embryo transfers are seldom reported in the literature. However, the use of Day 4 embryos in clinical practice increases flexibility when choosing the embryo transfer day, and enables avoiding embryo transfers on weekends. Methods and findings: We conducted a retrospective cohort study (1.1.2009-31.12.2014) comparing the clinical outcomes of fresh Day 3 (n=1371), Day 4 (n=360) and Day 5

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... 26) and vitrified-warmed Day 3 (n=635), Day 4 (n=175) and Day 5 (n=654) embryos. The live birth rates of fresh Day 3, Day 4 and Day 5 embryo transfers were 26.5%, 31.4% and 24.5%, respectively (p=0.026). The miscarriage rates were 16.6%, 11.7% and 23.0%, respectively (p=0.017). The live birth rates of Day 3, Day 4 and Day 5 vitrified-warmed embryo transfers were 22.0%, 30.9% and 22.6%, respectively (p=0.024). The miscarriage rates were 21.6%, 15.9% and 27.8%, respectively (p=0.049). Conclusions: The results of the present study show that Day 4 embryo transfers produce good clinical outcomes and can be considered for wider utilization at IVF clinics. Abstract Day 4 embryos should not be underestimated in IVF.