Explain NST with graph
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Non-Stress Test (NST)
What is the NST?
The Non-Stress Test (NST) is the most commonly used primary tool for antepartum fetal surveillance. It has been used to assess second and third trimester fetal well-being for over 40 years. The "non-stress" in its name means no uterine contractions or stressors are applied to the fetus - it simply monitors the spontaneous fetal heart rate (FHR) response to fetal movement.
The NST serves as a surrogate measure of two things:
- The developing fetal autonomic nervous system (ANS)
- The adequacy of uteroplacental function
Physiological Basis
When the fetal ANS is intact and the fetus is well-oxygenated, fetal movement triggers FHR accelerations through peripheral nerve receptor stimulation. This coupling of movement and acceleration is the foundation of the NST. A healthy fetus in active sleep will produce frequent, grouped movements - each paired with a corresponding heart rate rise.
How to Perform the NST
- The mother is placed in a semi-recumbent position
- An external cardiotocograph (CTG) is applied to continuously record the FHR
- No uterine contractions are induced
- Monitoring window: 20-40 minutes (may extend to 60-80 min if initially non-reactive to allow for fetal sleep cycles)
Interpretation: Reactive vs. Non-Reactive
Reactive NST (Normal / Reassuring)
- At least 2 FHR accelerations within a 20-minute window
- Each acceleration rises ≥15 beats/min above baseline
- Each acceleration lasts ≥15 seconds
For preterm fetuses <32 weeks: modified criteria apply - accelerations of ≥10 bpm for ≥10 seconds are acceptable, since younger fetuses have smaller but still meaningful accelerations.
Non-Reactive NST (Abnormal)
- Fails to meet the above criteria even after 40-60 minutes
- Does not automatically mean fetal jeopardy - fetal sleep state (which can last up to 40 minutes normally) is the most common cause
- Requires further evaluation: Biophysical Profile (BPP) or Contraction Stress Test (CST)
The NST Tracing - Figure 20-13
This is a reactive NST tracing showing FHR accelerations above a baseline of 130-140 bpm with good variability:
The upper channel shows the FHR (y-axis: beats/min, 30-240 scale). The upward spikes above the baseline represent accelerations associated with fetal movement. Good beat-to-beat variability (oscillations 6-25 bpm) is also visible - a sign of a healthy fetal nervous system.
FHR Variability on NST - What to Look For
Variability is one of the most important indicators of fetal well-being. Here are the four categories defined by NICHD guidelines:
| Category | Amplitude | Clinical Significance |
|---|---|---|
| Absent | Undetectable | Ominous - associated with fetal acidosis |
| Minimal | ≤5 bpm | Could reflect sleep, CNS depressants, or compromise |
| Moderate | 6-25 bpm | Normal, reassuring |
| Marked | >25 bpm | Possibly associated with hypoxia or cord compression |
| Sinusoidal | Smooth sine wave | Ominous - seen in fetal anemia |
The presence of decreased variability + late or severe variable decelerations together is an ominous combination.
Causes of Non-Reactive NST
| Cause | Notes |
|---|---|
| Fetal sleep state | Most common; lasts 20-40 min, rarely up to 2 hours |
| Extreme prematurity | Immature ANS |
| Maternal narcotics / CNS depressants | Cross placenta and suppress fetal CNS |
| Magnesium sulfate | Decreases FHR variability and accelerations |
| Fetal cardiac anomalies | Structural limitation on rate response |
| Fetal CNS anomalies | Disrupted autonomic coupling |
| True fetal compromise / hypoxia | Acidosis suppresses cardiac reflex responses |
Accuracy and Limitations
| Parameter | Value |
|---|---|
| False-negative rate (fetal death within 1 week of reactive NST) | ~1.9 per 1000 |
| % of term fetuses non-reactive at 30 min | ~10-12% |
| % still non-reactive at 40 min | <6% |
The NST is more specific than sensitive - it is a better indicator of fetal health than fetal illness. A reactive result is reassuring; a non-reactive result does not confirm compromise and must be followed up.
NST as Part of the Biophysical Profile (BPP)
In the BPP, the NST contributes 2 points out of a maximum score of 10:
| NST Score = 2 (Normal) | NST Score = 0 (Abnormal) |
|---|---|
| Moderate variability + accelerations coupled with fetal movements on a 30-min NST | FM and accelerations not coupled; insufficient, absent, or decelerative trace |
A BPP score of 8-10 is reassuring; 6 is suspicious; ≤4 is ominous and warrants immediate intervention.
Indications for NST
- Gestational diabetes mellitus
- Chronic hypertension / pre-eclampsia
- Post-term pregnancy (≥41 weeks)
- Intrauterine growth restriction (IUGR/FGR)
- Multiple gestation
- Prior fetal demise
- Decreased fetal movement reported by mother
- Oligohydramnios
For low-to-moderate risk conditions, weekly NST starting at 32 weeks is a common protocol. Higher-risk cases require earlier and more frequent testing.
Sources: Textbook of Family Medicine, 9e; Creasy & Resnik's Maternal-Fetal Medicine: Principles and Practice
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