Statistical analysis
Continuous variables were expressed as median (25% percentile, 75% percentile) and compared with Mann-Whitney U test; categorical variables were expressed as number (constituent ratio) and compared with χ² test, continuity correction χ² test or Fisher’s exact test between pregnant and none-pregnant groups. A two-sided α of less than 0.05 was considered statistically significant. Statistical analyses were done using the SAS software, version 9.4, unless otherwise indicated.