Statistical analysis
The descriptive data on participant characteristics are summarized using the mean and standard deviation for continuous variables. Counts and proportions are used for the categorical variables. Chi-square tests were performed to compare the categorical variables. To compare the continuous variables among the groups, an analysis of covariance (normally distributed variables) or a Kruskal-Wallis test (abnormally distributed variables) were used. For multivariate analyses, logistic regression was utilized. Crude odds ratios (OR) and adjusted ORs (aOR) with 95% confidence interval (CI) were calculated to assess the association between covariates and clinical outcomes. We selected the covariates on the basis of their associations with the outcomes of interest or a change in effect estimate of more than 10%. In the adjusted Model: we adjusted female age at embryo transfer, infertility duration, BMI, AFC, no. of oocyte retrieved, infertility type, protocol in the fresh cycle, single blastocyst transfer, endometrial thickness on the day of P administration, triple-line endometrial pattern, type of embryo transferred, no. of transferred embryos, no. of transferrable embryo and no. of good quality embryos transferred.
Interaction and stratified analyses were conducted according to female age at oocyte retrieval, female age at embryo transfer, infertility duration, infertility type, AFC, endometrial thickness on the day of P administration, fertilization type, protocol in the fresh cycle, no. of oocyte retrieved, no. of transferrable embryos, no. of transferred embryos, no. of good quality embryos transferred, type of embryo transferred, triple-line endometrial pattern and single blastocyst transfer. Data were analyzed with the use of the statistical packages R (The R Foundation; http://www.r-project.org;version 3.4.3) and Empower (R) (www.empowerstates.com,X&Y solutions, inc. Boston, Massachusetts). The level of significance was set at p < 0.05. The type I error rate was controlled using the Bonferroni stepdown procedure for multiple comparisons. When Bonferroni corrections were applied, unadjustedp = 0.05; the number of tests, 5 (live birth rate, miscarriage rate, clinical pregnancy rate, preterm birth rate, ectopic pregnancy rate); p = 0.01 with Bonferroni correction applied [0.05÷5]).