Comparisons of frozen-thawed D3/D5 and D3/D6 sequential embryo transfer in women with repeated implantation failure: a retrospective cohort study

Objective To investigate whether there are significant differences in pregnancy outcomes between frozen-thawed D3/D5 and D3/D6 sequential embryo transfer (SeET) in women with repeated implantation failure (RIF). Methods Women who experienced RIF and underwent frozen-thawed cycles with SeET were included and were divided into two groups: D3/D5 (n=455) and D3/D6 (n=308). Included cycles were stratified by the quality of transferred embryos and age. Results There were 252/455 (55.4%) clinical pregnancies in the D3/D5 group and 133/308 (43.2%) in the D3/D6 group (OR: 1.63, 95% CI 1.22 to 2.18). Rates for ectopic pregnancy, early miscarriage and multiple pregnancies did not differ between the two groups. In women under 35 years, with transfers of two good-quality embryos, the clinical pregnancy rate was significantly higher in the D3/D5 group compared with the D3/D6 group (65.6% vs 42.4%, OR: 2.59, 95% CI 1.41 to 4.76). This trend persisted when analysing good-quality cleavage-stage embryo and poor-quality blastocyst (D3/D5) transfers versus two good-quality embryo (D3/D6) transfers (61.2% vs 42.4%, OR: 2.14, 95% CI 1.11 to 4.12). For women aged 35 years or older, pregnancy outcomes did not differ. Conclusion The clinical pregnancy rate was significantly higher in the D3/D5 group compared with the D3/D6 group. When contrasting the outcomes of transferring a good-quality cleavage-stage embryo and a poor-quality blastocyst in the D3/D5 group with transferring two good-quality embryos in the D3/D6 group, the clinical pregnancy rate was significantly higher in the D3/D5 group. However, among women aged 35 years or older, there were no significant differences in pregnancy outcomes.