Statistical analyses
One sample of the Kolmogorov–Smirnov test was used to test the normal distribution of continuous variables. Continuous variables were given as mean ± SD if normally distributed, and as median (interquartile range) if not normally distributed. Statistical comparison was carried out by Student’s t-test, Mann–Whitney U-test for continuous variables and chi-square test for categorical variables, where appropriate. And logistic regression analysis was used to analyse factors predicting the ongoing pregnancy rate. Statistical analysis was performed using the Statistical Program for Social Sciences (SPSS Inc., Version 24.0, Chicago, USA). The two-tailed value of P<0.05 was considered statistically significant.