Study site
The subjects were expectant ladies enrolled in Guizhou Province from
March to April 2020. Utilizing a cross-sectional examination by
stratified sampling strategy, Guizhou Province was partitioned into 7
strata as indicated by Guiyang, Qiannan Prefecture, Anshun, Zunyi,
Bijie, Tongren, and Liupanshui. Two public medical clinics were randomly
chosen inside each metropolitan zone, while 2-3 public medical clinics
were arbitrarily chosen inside each countryside zone.