What is a normal non-stress test (NST) reading in pregnancy?

Normal Non-Stress Test (NST) Reading in Pregnancy

A normal (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 at ≥32 weeks' gestation. 1, 2

Defining a Reactive NST

At ≥32 Weeks Gestation

• Two or more accelerations of the fetal heart rate are required 1, 2
• Each acceleration must be ≥15 beats/minute above baseline 1, 2
• Each acceleration must last ≥15 seconds 1, 2
• These criteria must be met within a 20-minute observation period 1, 2

Before 32 Weeks Gestation

• Two or more accelerations of at least 10 beats/minute above baseline 2
• Each acceleration must last at least 10 seconds 2
• This reflects developmental differences in fetal autonomic function 2

Clinical Significance of a Reactive NST

Predictive Value

• A reactive NST has a negative predictive value >99.9% for fetal well-being 1
• Stillbirth risk within 1 week is only 0.8 per 1,000 cases (0.08%) after a reactive NST 1
• This makes it a highly reliable predictor of fetal well-being for up to one week 3, 4

What a Reactive NST Indicates

• Immediate fetal oxygenation and acid-base balance are normal 1
• The fetal autonomic nervous system is functioning appropriately 4
• The test serves as a surrogate measure of the developing fetal autonomic nervous system 4

Testing Protocol

Duration and Extension

• Begin with 20 minutes of continuous observation 2
• If nonreactive after 20 minutes, extend monitoring to 40 minutes before declaring it nonreactive 2
• The extended timeframe accounts for fetal sleep cycles, which typically last 20-40 minutes 2

Technical Setup

• Use a cardiotocograph device that records fetal heart rate via continuous-wave Doppler ultrasound 2
• Position the external transducer on the maternal abdomen to obtain optimal fetal heart rate signal 2
• Monitor with the patient lying on her left side 5

Critical Limitations

What NST Cannot Predict

• No antenatal test, including NST, can predict stillbirth related to acute events such as placental abruption or cord accidents, regardless of test frequency 1, 6, 2
• NST provides assessment of immediate fetal well-being only, not chronic placental function 1

Common Pitfalls to Avoid

• Fetal sleep cycles are the most common cause of nonreactive NST 2
• Always extend monitoring to 40 minutes before declaring a test nonreactive to avoid false positives 2
• Consider fetal acoustic stimulation to awaken the fetus and reduce unnecessary interventions 2
• Do not use NST in low-risk pregnancies, as routine testing can cause iatrogenic prematurity from false-positive results 1, 2

When to Use NST

Appropriate Populations

• Reserve NST surveillance exclusively for high-risk pregnancies 1, 2
• High-risk conditions include: advanced maternal age, obesity, hypertensive disorders, diabetes, chronic renal disease, thyroid disorders, thrombophilia, connective tissue disease, history of unexplained stillbirth, intrauterine growth restriction (IUGR), decreased fetal movement, multiple gestations, fetal arrhythmias, and amniotic fluid abnormalities 1

Timing and Frequency

• Initiate antepartum fetal surveillance at 32-34 weeks' gestation for most high-risk conditions 1, 2
• Weekly or twice-weekly testing has become standard clinical practice in high-risk pregnancies 1, 2
• More frequent testing does not prevent stillbirth from acute events 2

Enhanced Assessment: Modified Biophysical Profile

Combining NST with Amniotic Fluid Assessment

• A modified BPP combines NST (acute assessment) with amniotic fluid volume (chronic assessment) 1
• This provides more comprehensive evaluation than NST alone 2
• Normal amniotic fluid is defined as maximum vertical pocket (MVP) ≥2 cm throughout gestation 1
• Alternatively, amniotic fluid index (AFI) ≥5 cm at ≥37 weeks or ≥8 cm at <37 weeks is considered normal 1