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 < 1.5 were defined as grade D, and those with a BNR of ≥ 1.0 and < 1.2 were defined as grade E. Using this proposed grading model, 5460 embryos with known implantation data were retrospectively analyzed after D4 transfer in FET cycles. The transferred embryos exhibited a similar declining trend in implantation and live birth rates from the top grade A to the lowest grade E. The in vitro fertilization group showed increased implantation rates of grade B and E embryos compared with the intracytoplasmic sperm injection group (grade B: 41.99%, 34.63%, χ2 = 5.84, p < 0.05 and grade E: 18.98%, 14.08, χ2 = 75.62, p < 0.01). Receiver-operating characteristic analysis revealed that our proposed model predicted the implantation outcomes and live birth rates of all embryos (area under the curve = 0.65; 95% confidence interval [CI],0.63-0.66; p < 0.01 and AUC = 0.73; 95%CI, 0.65-0.84; p < 0.001, respectively). This study demonstrates that the new grading system provided by us can be a useful tool for assisting embryo selection via changes in embryo morphology. D4 embryo transfer provides a simple and applicable method for FET cycles in daily practice.
Keywords: Compaction; Embryo; Grading system; Implantation.