Statistical analysis
Data are presented as mean and standard deviation (continuous variables) or counts and proportions (categorical variables). One-way analysis of variance was used for continuous variables, and the Pearson’s χ2 test was used for categorical variables with Fisher’s exact test when necessary. We performed logistic regression to explore the effects of BMI on live birth after accounting for the following potential confounders: female age, infertility duration, endometrium (EM) thickness, no. of oocyte retrieved, infertility type, protocol in fresh cycle (agonist, antagonist, other); EM protocol (natural cycle, HRT, GnRHa+HRT). We calculated crude odds ratios (OR) and adjusted ORs (aOR) with 95% confidence interval (CI). Interaction and stratified analyses were conducted according to confounders. 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.