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.
Cox proportional hazard model was used to evaluate the relative prognostic significance of female age, BMI, the number of retrieved oocytes and the primary diagnosis of infertility in relation to CLBR. Regression analyses were made for the individual treatment groups in all FET cycles with transfer to evaluate the impact of independent variables on the total LBRs from FET (n = 919).
All pregnancies within 18 months from ovarian stimulation were analyzed, whether achieved by fresh or frozen IVF cycle. The Kaplan-Meier method was used to calculate the cumulative proportion of ongoing pregnancies leading to live births, and time to pregnancy was graphically depicted by cumulative incidence curves. The log-rank test was used to measure whether significant differences existed in the cumulative incidence curves. Patients who did not reach the primary outcome (live birth) including those achieved a continuing pregnancy that did not lead to live birth were censored. 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.