Abstract: Objective To investigate the effect of sampling time and site on the pH value of umbilical cord venous blood. 
Methods A total of 170 pregnant women who met the criteria were randomly selected and included in the study. Free and connected placental cord venous blood were collected at 5 min, 15 min and 30 min after fetal delivery, and the pH of the corresponding blood samples was measured. 
Results With the extension of sampling time, the pH value of the free segment and the connected placenta segment decreased gradually; the pH value of the free segment was significantly higher than that of the connected placenta segment, and there were significant differences in interaction between groups, time points and time points between groups (P＜0.05). 
Conclusion With the extension of sampling time of cord blood, the pH value of umbilical vein blood decreases gradually. The pH value of free segment of umbilical venous blood is higher than that with connecting placental segment. Therefore, in order to ensure that the measurement results of pH value of umbilical venous blood can better reflect the actual physiological situation of the fetus, umbilical cord blood sampling should be conducted as soon as possible, and sampling should be conducted in the free segment far from the placenta.

Key words: umbilical cord, venous blood, cordocentesis, hydrogen ion concentration