Non-Stress Test Protocol for Fetal Monitoring

Technical Performance of NST

The American College of Radiology recommends performing NST by monitoring fetal heart rate for 20 minutes initially, extending to 40 minutes if needed, with a reactive test defined as two or more fetal heart rate accelerations of 15 beats/minute above baseline lasting 15 seconds at ≥32 weeks' gestation (or 10 beats/minute for 10 seconds before 32 weeks) 1
Equipment and setup for NST should include a cardiotocograph device that records fetal heart rate via continuous-wave Doppler ultrasound along with uterine activity 1
Position the external transducer on the maternal abdomen to obtain optimal fetal heart rate signal 1
Begin with 20 minutes of continuous observation, and if the test is nonreactive after 20 minutes, extend monitoring to 40 minutes before declaring it nonreactive 1
The extended timeframe accounts for fetal sleep cycles, which typically last 20-40 minutes 2

At ≥32 weeks gestation, a reactive NST is defined as two or more fetal heart rate accelerations of at least 15 beats/minute above baseline, lasting at least 15 seconds, within a 20-minute observation period 1
Before 32 weeks gestation, a reactive NST is defined as two or more accelerations of at least 10 beats/minute above baseline, lasting at least 10 seconds, reflecting developmental differences in fetal autonomic function 1
A nonreactive NST is defined as one or no acceleration meeting the above criteria after 40 minutes of observation 1

Begin antepartum fetal surveillance at 32-34 weeks' gestation for most high-risk conditions 1
Individualize timing based on the specific indication, gestational age, and likelihood of neonatal survival 1
Weekly or twice-weekly testing has become standard clinical practice, though this frequency is not based on rigorous scientific evidence 1, 3
More frequent testing does not prevent stillbirth from acute events such as placental abruption or cord accidents 1, 3

Fetal sleep cycles are the most common cause of nonreactive NST 2
Always extend monitoring to 40 minutes before declaring a test nonreactive 1
Consider fetal acoustic stimulation to awaken the fetus and reduce unnecessary interventions 3
Antenatal fetal surveillance in low-risk women has the potential to cause iatrogenic prematurity secondary to preterm delivery for false-positive results 1
Reserve NST for pregnancies with specific risk factors for fetal compromise 1
NST cannot predict stillbirth from acute events like placental abruption or cord accidents 1, 3
Solid evidence that current surveillance strategies effectively prevent antenatal stillbirth is lacking 3
Consider combining NST with amniotic fluid assessment (modified biophysical profile) for more comprehensive evaluation 1