Maternal characteristics
A total of 23726 FET cycles were included in this study, of which, 763 cycles were single cleavage-stage embryo transfers (C-1 group), 13004 were double cleavage-stage embryo transfers (C-2 group), 7913 were single blastocyst-stage embryo transfers (B-1 group), and 2046 were double blastocyst-stage embryo transfers (B-2 group). Maternal characteristics for all FET cycles and for each group are summarized in Table 1. Maternal characteristics among the four groups were statistically different for maternal age, BMI, infertility type, cause of infertility, duration of infertility, type of fertilization in the current cycle, and previous ART cycles (P<0.05). Results of pairwise comparisons of maternal characteristics between the groups are shown in Table S1.