Fetal Non-Stress Test: Assuring vs Non-Assuring Results
Immediate Management Based on NST Result
A reactive (assuring) NST is highly predictive of fetal well-being with a negative predictive value >99.9%, while a non-reactive (non-assuring) NST requires immediate further evaluation with either a full biophysical profile or modified biophysical profile to determine if delivery is indicated. 1
Defining Assuring (Reactive) NST
Interpretation Criteria
- At ≥32 weeks: Two or more fetal heart rate accelerations of ≥15 beats/minute above baseline, lasting ≥15 seconds, within 20 minutes 2
- Before 32 weeks: Two or more accelerations of ≥10 beats/minute above baseline, lasting ≥10 seconds 2
- Testing duration: Begin with 20 minutes of monitoring; extend to 40 minutes before declaring non-reactive to account for fetal sleep cycles 2
Clinical Significance of Reactive NST
- Stillbirth risk within 1 week: Only 0.8 per 1,000 cases (0.08%) 1
- Negative predictive value: 91.2% for healthy fetus outcome 3
- Specificity: 85.4% for identifying truly healthy fetuses 3
Defining Non-Assuring (Non-Reactive) NST
Interpretation Criteria
- Definition: One or no acceleration meeting reactive criteria after 40 minutes of observation 2
- Most common cause: Fetal sleep cycles (not fetal compromise) 2
Clinical Significance of Non-Reactive NST
- Sensitivity: Only 40.9% for identifying compromised fetuses 3
- Positive predictive value: Only 28.1% for adverse outcomes 3
- Key limitation: Non-reactive NST identifies a population at risk but cannot stand alone for decision-making 3
Management Algorithm for Non-Reactive NST
Step 1: Immediate Further Testing
Proceed immediately to modified biophysical profile (NST + amniotic fluid assessment) or full biophysical profile 1, 4
Step 2: Interpret Combined Results
If BPP Score 8-10 (Reassuring)
- Preterm (<37 weeks): Continue surveillance with increased frequency (twice weekly) 1
- Term (≥37 weeks): Consider delivery, especially if other risk factors present 1, 5
- Active labor at term: Proceed with delivery with continuous electronic fetal monitoring 5
If BPP Score ≤6 (Non-Reassuring)
- Any gestational age at term: Deliver immediately 1
- Preterm: Individualize based on gestational age, but strongly consider delivery if viable 1
If Oligohydramnios Detected (MVP <2 cm or AFI <5 cm)
- Term pregnancy: Deliver 1
- Preterm: Increase surveillance frequency and consider delivery based on severity 1
Step 3: Additional Doppler Assessment if Growth Restriction Suspected
- Umbilical artery Doppler: Absent or reversed end-diastolic flow predicts adverse outcomes with >20% perinatal death rate 1
- If abnormal Doppler at term: Deliver immediately 1
- If abnormal Doppler preterm: Daily or twice-daily monitoring until delivery indicated 1
Management at Term (≥37 Weeks) with Abnormal Testing
Delivery Indications
At term gestation, abnormal antenatal test results are an indication for delivery, including: 1
- BPP score ≤6 1
- Modified BPP with non-reactive NST 1, 5
- Oligohydramnios (MVP <2 cm or AFI <5 cm) 1
- Polyhydramnios (MVP >8 cm or AFI >25 cm) 1
- Abnormal umbilical artery Doppler findings 1
Mode of Delivery
- Induction of labor: Reasonable first option if no contraindications 1
- Cesarean delivery: May be warranted due to concerns about fetal intolerance of labor 1, 5
- Continuous electronic fetal monitoring: Required throughout labor 5
Critical Limitations of All Antenatal Testing
What NST and BPP Cannot Predict
No antenatal test can predict stillbirth from acute events, regardless of test frequency or type: 4, 2, 6
- Cord accidents: Account for majority of false-negative reactive NSTs 6
- Placental abruption: Occurs acutely after normal testing 6
- Congenital anomalies: May not manifest on heart rate testing 6
False-Negative Rate
- Fetal death within 7 days of reactive NST: 0.026% (4 deaths in 1,564 deliveries) 6
- Presumed causes: Cord accidents (75%), placental abruption (25%) 6
High-Risk Populations Requiring NST Surveillance
Indications for Antenatal Testing (Start at 32-34 Weeks)
Reserve NST for high-risk pregnancies only; routine testing in low-risk pregnancies causes iatrogenic prematurity from false-positive results: 1, 4, 2
- Advanced maternal age 4
- Obesity 4
- Hypertensive disorders 4
- Diabetes (gestational or pregestational) 4
- Chronic renal disease 4
- Thyroid disorders 4
- Thrombophilia 4
- History of unexplained stillbirth 4
- Intrauterine growth restriction 4
- Decreased fetal movement 4
- Multiple gestations 4
- Fetal arrhythmias 4
- Oligohydramnios or polyhydramnios 4
Testing Frequency
- Standard practice: Weekly or twice-weekly testing 4, 2
- Evidence base: Optimal interval lacks rigorous scientific evidence 4, 2
- Critical situations: Daily or more frequent testing may be indicated 1
Common Pitfalls to Avoid
Technical Errors
- Declaring non-reactive too early: Always extend monitoring to 40 minutes before declaring non-reactive 2
- Not accounting for fetal sleep cycles: Most common cause of non-reactive NST 2
- Consider fetal acoustic stimulation: Can awaken fetus and reduce unnecessary interventions 2
Clinical Decision-Making Errors
- Using non-reactive NST alone for delivery decision: Always perform additional testing (BPP or modified BPP) 1, 3
- Testing low-risk pregnancies: Causes more harm than benefit through false-positive results 1, 4, 2
- Over-reliance on normal testing: Cannot predict acute events; maintain clinical vigilance 4, 2, 6
- Ignoring amniotic fluid assessment: Oligohydramnios is independent risk factor for stillbirth (OR 2.6) 1