Statistical analysis
The data on continuous variables with normal distribution were presented as means and standard deviations (SD). The concentration of 25(OH)D was described as a continuous variable and categorized following the standard of the Committee of the IOM (sufficient: ≥50nmol/L; insufficiency:30-49.9nmol/L; deficiency:<30nmol/L). According to the scientific literature and biological plausibility, we selected covariables as possible confounders of the association between vitamin D concentrations and adverse pregnancy outcomes we studied, including maternal age and parity, since these were hypothesized to be strongly related to vitamin D status[17].Association between 25(OH)D levels and adverse pregnancy outcomes was assessed by logistic regressions including these potential confounders. Logistic regression was also used to estimate adjusted odds ratios (OR) for adverse pregnancy outcomes during the groups. All OR were presented with 95% confidence intervals (CIs). SPSS (version 22.0; IBM Corp, Armonk, NY) was used for all calculations. In addition, Women with one or more of the adverse pregnancy outcomes were defined as “women with adverse pregnancy outcome group”, and the others were defined as “women without adverse pregnancy outcome group”. Figure 2 was done using Graphpad Prism Version 9.00 software. Plot the frequency distribution of vitamin D concentrations as a statistical graph, followed by a Gaussian fit.