The fetal response to changes in the composition of maternal inspired air in human pregnancy.

The effect of hypoxia upon the fetus is dependent upon not only the degree of hypoxia induced but probably also upon gestational age and the initial level of fetal oxygenation. Mild hypoxia (12% or over) causes fetal tachycardia, while a more severe insult may cause bradycardia. The effect of hypoxia upon FBM in human pregnancy is uncertain, but depending upon the severity of the hypoxia, it is likely that FBM is reduced or abolished. Hyperoxia has little effect upon the fetal heart rate or FBM in normal circumstances, but an increase in FBM occurs in the presence of fetal hypoxia. No significant change in fetal heart rate in human pregnancy occurs during hypercapnia which is, however, a potent stimulus to fetal breathing. On the other hand, hypocapnia caused by hyperventilation is associated with a decrease in FBM with no obvious change in fetal heart rate. Smoking is associated with a fetal tachycardia and a decrease in FBM. The decrease in FBM is small, the effect being maximal at 30 min after smoking with recovery by 90 min.