statistical analysis
All statistical analysis was conducted by SPSS 25.0 (SPSS, Chicago, USA). The normality of continuous variables was assessed by Shapiro-Wilk test. If conforming to normality distribution, mean±SD and analysis of variance(ANOVA) was adopted for variables description and intra-group comparisons. Vice versa, if non-normality distribution was ascertained, median (25th-75th percentile) and Mann-Whitney U test was used for variables description and intragroup comparisons. Frequencies (percentages) were utilized to describe categorical variables. The comparisons of categorical variables were evaluated by χ2 test or Fisher’s exact test. Stratification analysis was conducted on pregnancy outcomes in fresh ET and FET cycles. Among the pairwise comparisons of four subgroups, Bonferroni method was used to adjust p-values. In order to account for the differences among subgroups, age, BMI, duration of infertility, types of infertility, AFC, AMH, FSH, initial uterine diameter and dysmenorrhea history were included in logistic regression model. P<0.05 was considered as statistical significance.