How to interpret a non-stress test (NST) in pregnancy?

How to Interpret a Non-Stress Test (NST)

Definition and Basic Interpretation

A reactive NST is defined as two or more fetal heart rate accelerations of ≥15 beats/minute above baseline, lasting ≥15 seconds, within a 20-minute observation period at ≥32 weeks gestation (or ≥10 beats/minute for ≥10 seconds before 32 weeks). 1

Technical Performance

• Monitor fetal heart rate continuously for 20 minutes initially using a cardiotocograph device with continuous-wave Doppler ultrasound 1
• Position the external transducer on the maternal abdomen to obtain optimal fetal heart rate signal 1
• If nonreactive after 20 minutes, extend monitoring to 40 minutes before declaring the test nonreactive 1
• The extended timeframe accounts for fetal sleep cycles, which typically last 20-40 minutes and are the most common cause of nonreactive NST 1

Interpretation Categories

Reactive (Normal) NST

• Two or more accelerations meeting criteria within 20 minutes 1
• Highly predictive of fetal well-being with negative predictive value >99.9% 2
• Stillbirth risk within 1 week is only 0.8 per 1,000 cases (0.08%) 2
• Associated with favorable fetal outcomes in 85-89% of high-risk pregnancies 3, 4

Nonreactive (Abnormal) NST

• One or no acceleration meeting criteria after 40 minutes of observation 1
• Requires immediate further evaluation—never use nonreactive NST alone for delivery decisions 2
• Associated with significantly increased cesarean section rates for fetal distress and higher perinatal mortality 3

Management Algorithm Based on NST Result

If Reactive NST

• Continue surveillance at established intervals (weekly or twice-weekly in high-risk pregnancies) 2, 1
• Combine with amniotic fluid assessment to create modified biophysical profile for comprehensive evaluation 2
• No additional testing needed unless clinical situation changes 2

If Nonreactive NST

• Proceed immediately to modified biophysical profile (NST + amniotic fluid) or full biophysical profile 2
• If BPP score 8-10: Continue surveillance with increased frequency (twice weekly) for preterm pregnancies 2
• If BPP score ≤6: Deliver immediately, regardless of gestational age 2
• If oligohydramnios detected: Deliver at term 2
• At term gestation with nonreactive NST: This is an indication for delivery 5, 2

What NST Actually Measures

• NST provides real-time assessment of acute fetal oxygenation and acid-base balance 2
• It does NOT assess chronic placental function (requires amniotic fluid assessment) 2
• It does NOT evaluate fetal growth (requires biometry) 2
• NST cannot predict stillbirth from acute events like placental abruption or cord accidents 2, 6

Critical Clinical Context

When to Use NST

• Reserve for high-risk pregnancies only—routine testing in low-risk pregnancies causes iatrogenic prematurity from false-positive results 2, 1
• Initiate at 32-34 weeks gestation for most high-risk conditions 2, 1
• High-risk indications include: hypertensive disorders, diabetes, IUGR, decreased fetal movement, multiple gestations, amniotic fluid abnormalities, advanced maternal age, chronic renal disease, history of unexplained stillbirth 2

Frequency of Testing

• Weekly or twice-weekly testing is standard clinical practice, though optimal interval lacks rigorous scientific evidence 2, 1
• Daily or more frequent testing may be indicated in critical situations like growth restriction or abnormal Doppler findings 2

Common Pitfalls to Avoid

• Never declare NST nonreactive before 40 minutes of monitoring—fetal sleep cycles are the most common cause of apparent nonreactivity 1
• Consider fetal acoustic stimulation to awaken the fetus and reduce unnecessary interventions 1
• Do not rely on NST alone when nonreactive—always perform additional testing (BPP or modified BPP) before making delivery decisions 2
• Pay close attention to fetal heart rate decelerations on NST, as they may identify fetuses at risk for cord accidents 6
• Remember that false negatives occur in approximately 0.026% of cases, typically associated with cord accidents, abruptio placentae, or congenital anomalies 6

Special Considerations

Modified Biophysical Profile

• Combining NST with amniotic fluid assessment creates a modified BPP that evaluates both acute (NST) and chronic (amniotic fluid) markers of fetal well-being 2
• This is the recommended approach rather than NST alone for comprehensive fetal assessment 5, 2

Term Pregnancy Management

• At term with abnormal NST (nonreactive), delivery is indicated 5, 2
• Induction of labor is reasonable if no contraindications exist, but cesarean delivery may be warranted due to concerns about fetal well-being and intolerance of labor 5, 7