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.