A New Day 4 Grading System to Assess Embryo Quality in Frozen Embryo Transfer Cycles

To present a new day 4 (D4) embryo grading system to assess embryos in frozen–thawed embryo transfer (FET) cycles. A new grading system (grades A–E) was developed from the 2011 ESHRE Istanbul Consensus for D4 embryos in FET cycles. Embryos with complete compaction were classified as grade A; those with partial compaction were assigned as grade B; and those without compaction were classified as grades C, D, and E according to their different blastomere number ratio (BNR; number of embryo blastomeres on D4/number of embryo blastomeres on D3, D4/D3). Embryos with a BNR of ≥ 1.5 were defined as grade C, those with a BNR of ≥ 1.2 and