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.