Statistical analyses
Statistical analyses were performed using 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, Fisher’s exact test, one-way analysis of variance (ANOVA) or Bonferroni testas appropriate. Logistic regression analysis was performed to explore the effects of different blastocyst transfer strategies on live birth after controlling for potential confounders including female age, female BMI, duration of infertility and endometrial thickness. A two-sided P < 0.05 was considered to be statistically significant.