Statistical analysis
All statistical analysis were performed in SPSS version 22 (SPSS, Chicago, IL). Data were expressed as mean ± standard deviation (SD) or median (interquartile range) for continuous variables based on the distribution; and counts with proportions for categorical variables. Box plots were used to display the levels of sFlt-1/PlGF ratios in different subsets of patients. one-way ANOVA or Kruskal-Wallis test were used for comparing group differences. Receiver operating characteristic (ROC) curve was used to illustrate the diagnostic performance and the cutoffs were identified as the value corresponding to the largest Youden index [sensitivity-(1-specificity)]. P values <0.05 was used to indicate significance for two-tailed statistical test results.