What is the protocol for fetal monitoring on a Non-Stress Test (NST)?

Non-Stress Test (NST) Protocol for Fetal Monitoring

For antepartum fetal surveillance, perform 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

Technical Performance of NST

Equipment and Setup:

• Use 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

Testing Duration:

• Begin with 20 minutes of continuous observation 1
• If the test is nonreactive after 20 minutes, extend monitoring to 40 minutes before declaring it nonreactive 1
• This extended timeframe accounts for fetal sleep cycles, which typically last 20-40 minutes 1

Interpretation Criteria

Reactive (Normal) NST:

• At ≥32 weeks gestation: 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: 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

Nonreactive (Abnormal) NST:

• One or no acceleration meeting the above criteria after 40 minutes of observation 1
• This requires further evaluation, typically with a contraction stress test (CST) or biophysical profile (BPP) if not contraindicated 2

Note on acceleration duration criteria: While some protocols require the heart rate to remain elevated at 15 bpm above baseline for the entire 15 seconds (long criterion), others simply require 15 seconds from acceleration onset to return to baseline (short criterion) 3. The short criterion identifies more reactive tests (89% vs 79%) without compromising predictive value for poor outcomes 3.

Adjunctive Techniques to Improve Efficiency

Fetal Acoustic Stimulation:

• Apply a 2,000 Hz pure-tone auditory stimulus at 105-120 decibels for 5 seconds to elicit fetal heart rate acceleration 4
• This technique can shorten testing time and reduce false-positive nonreactive results by awakening a sleeping fetus 1, 4
• When fetal heart rate acceleration exceeds 15 beats/minute in response to acoustic stimulation, subsequent contraction stress tests are consistently negative 4

Maternal Recording of Fetal Movement:

• Women may be asked to record when fetal movement occurs during NST, though movement recognition is not required for test interpretation 1
• The theoretical basis of NST is that a healthy fetus increases heart rate in response to movement 1

Clinical Context and Timing

Initiation of Testing:

• 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

Testing Frequency:

• Weekly or twice-weekly testing has become standard clinical practice, though this frequency is not based on rigorous scientific evidence 1
• More frequent testing does not prevent stillbirth from acute events such as placental abruption or cord accidents 1

Predictive Value and Limitations

Reassuring Results:

• A reactive NST is highly predictive of fetal well-being, with perinatal death occurring in only 1.0% of cases within one week of a reactive test 2
• The negative predictive value is 98%, making it a reliable indicator of good outcome 5
• A reactive NST is as predictive of good outcome as a negative contraction stress test 2

Nonreactive Results:

• The positive predictive value for poor outcome is only 11%, meaning most nonreactive tests are false positives 5
• Sensitivity is 50% and specificity is 88% for detecting fetal compromise 5
• Nonreactive NST requires further evaluation with contraction stress test, biophysical profile, or Doppler velocimetry 5, 2

Common Pitfalls to Avoid

False-Positive Nonreactive Tests:

• Fetal sleep cycles are the most common cause of nonreactive NST 1
• Always extend monitoring to 40 minutes before declaring a test nonreactive 1
• Consider fetal acoustic stimulation to awaken the fetus and reduce unnecessary interventions 1, 4

Inappropriate Use in Low-Risk Pregnancies:

• 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

Overreliance on NST Alone:

• NST cannot predict stillbirth from acute events like placental abruption or cord accidents 1
• Solid evidence that current surveillance strategies effectively prevent antenatal stillbirth is lacking 1
• Consider combining NST with amniotic fluid assessment (modified biophysical profile) for more comprehensive evaluation 1