Statistical analysis
REGRESSION, INTERACTION
The analyses in the current study were conducted according to the intention-to-treat principle. The normality of continuous variables was tested by Kolmogorov–Smirnov test. For non-normally distributed continuous variables, the means (25th-75th percentiles) were displayed. Categorical parameters were presented as frequencies (percentages). Kruskal-Wallis test for continuous variables and Pearson’s χ2 test or Fisher’s exact test for qualitative data were performed, where appropriate. Univariate analysis was performed to examine the relationship between hCG treatment and clinical outcomes. Multivariate logistic regression analysis was conducted to adjust potential confounders and to further identify the association of hCG treatment with clinical outcomes in AC-FET cycles. Interaction and stratified analyses were performed according to protocol in the FET cycle (AC and GnRH-a+AC), type of embryo transferred (cleavage stage and blastocyst stage), number of embryos transferred (1 and 2) and number of good quality embryos transferred (0,1 and 2). A 