Sleep cycles: The fetus may have sleep cycles 20 to 80 minutes in duration. During these periods, the long-term variability of the fetal heart rate is decreased and the tracing is likely to be nonreactive. To rule out sleep cycle as a cause for a nonreactive NST, prolonged monitoring (longer than 80 minutes at times) or VAS may be required.

Medications: Certain maternal medications cross the placenta and can have an effect on the fetal heart rate, movement, and amniotic fluid volume. There are a number of medications administered in the management of labor and complications of labor that can have an influence on the tests for fetal well-being. Glucocorticosteroids given for the purpose of enhancing fetal maturity have been shown to influence BPP scores by decreasing the amniotic fluid index, decreasing fetal movement, and decreasing breathing motion. Magnesium sulfate can decrease the fetal heart rate variability. Other medications, such as narcotics, sedatives, and β-blockers have been shown to decrease fetal heart rate variability and reactivity.

Maternal smoking and illicit drugs: The maternal use of illicit drugs and smoking results in a transient decrease in fetal heart rate variability.

Maternal hypoglycemia: Maternal hypoglycemia may reduce fetal heart rate variability as well as fetal movement and breathing.