The fetal heart rate response to acute hypoxemia was observed during an accidental drop in oxygen concentration inspired by the mother in labor. There was a marked increase in FHR variability in the early stage of hypoxia and also during the recovery phase. While an increase in fetal heart variability is generally considered to be a normal physiological event, it may also indicate a state of fetal hypoxemia associated with an intact fetal circulation, and should be treated with caution during labor.