Statistical analyses
Statistical analyses were performed with SPSS 22.0 (IBM, Armonk, USA). Quantitative variables were presented as means ± standard deviations and categorical variables were expressed as frequencies and percentages. The data were analyzed using the Chi-squared test, one-way analysis of variance (ANOVA) or Bonferroni test as appropriate. Logistic regression analysis was performed to explore the effects of post-thawed culture period on live birth after controlling for potential confounders including female age, female BMI, endometrial preparation, type of infertility and duration of infertility. A two-sided P -value < 0.05 was considered to be statistically significant.