What the Fetal Non-Stress Test (NST) Assesses
The NST assesses acute fetal well-being by evaluating the integrity of the fetal autonomic nervous system through monitoring fetal heart rate accelerations in response to fetal movement. 1, 2
Primary Assessment Parameters
The NST specifically evaluates:
Fetal heart rate accelerations - The presence of at least 2 accelerations of ≥15 beats/minute above baseline, lasting ≥15 seconds, within a 20-minute period (at ≥32 weeks gestation) indicates a reactive test and confirms adequate fetal oxygenation and acid-base balance 1, 2
Fetal autonomic nervous system function - Heart rate accelerations serve as a surrogate measure of the developing fetal autonomic nervous system, with reactivity indicating intact central nervous system function 3
Immediate fetal oxygenation status - The NST provides real-time assessment of acute fetal well-being, distinguishing it from chronic markers like amniotic fluid volume 4, 1
What the NST Does NOT Assess
Critical limitation: The NST cannot predict stillbirth from acute catastrophic events such as placental abruption or cord accidents, regardless of how recently the test was performed or how frequently it is repeated. 1, 2
The NST alone does not evaluate:
- Amniotic fluid volume - This requires separate ultrasound assessment and reflects chronic placental function over the preceding week 4, 1
- Fetal growth - Biometry is needed to assess for growth restriction 4
- Fetal anatomy or structural abnormalities 1
- Placental function beyond acute oxygenation 1
Clinical Performance Characteristics
High negative predictive value (>99.9%) - A reactive NST is highly reassuring, with stillbirth risk of only 0.8 per 1,000 cases (0.08%) within one week 1
Better specificity than sensitivity - The NST is more reliable at confirming fetal health than detecting fetal compromise, making it an excellent screening tool but requiring additional testing when nonreactive 3
False negatives are uncommon but possible - Fetal deaths have occurred within 7 days of reactive NSTs (0.026% in one series), typically from cord accidents or abruption 5
Modified Biophysical Profile Context
When combined with amniotic fluid assessment, the NST becomes a "modified biophysical profile" that evaluates both acute (NST) and chronic (amniotic fluid) markers of fetal well-being. 4, 1 This combination is the recommended approach rather than NST alone, as oligohydramnios is an independent risk factor for stillbirth that the NST cannot detect 1.
Common Pitfall
The most frequent cause of a nonreactive NST is fetal sleep cycles (lasting 20-40 minutes), not fetal compromise 2. Always extend monitoring to 40 minutes before declaring a test nonreactive, and consider fetal acoustic stimulation to awaken the fetus 2.