Statistical analysis
All analyses have been performed using IBM Spss statistics 21.For continuous variables, Student’s t-test and Mann–Whitney test were used for data with homogeneous variance and heterogeneous variance respectively. The x2 test was used for categorical variables. Logistic regression analyses were conducted to identify independent correlates between each possible confounding factor, especially protocols for endometrium preparation and pregnancy outcome after adjusting for other confounders that were identified in our univariate analysis. A p-value <0.05 was considered statistically significant.