Statistical analysis
Categorical variables were expressed as the number of cases (n) and percentage of occurrence (%). The numerical data are presented as the mean ±SEM and differences in pa­rameters between the groups were compared using inde­pendent sample t-tests or χ2 tests as appropriate. Univariate and multivariate regression analyses were performed to determine the association between education level and live birth, excluding the interference of confounding factors. If adding a covariate to the model changed the estimated effect of a woman’s educational level on live birth by 10% or more, the covariate was considered a confounder of the relationship between the woman’s educational level and live birth and was included in the final multivariable models. We calculated crude odds ratios (OR) and adjusted ORs (aOR) with a 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.