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Non-Stress Test (NST) - Obstetrics & Gynecology
Definition & Principle
The NST is the most commonly used primary tool in antepartum fetal surveillance. It monitors fetal heart rate (FHR) patterns in response to fetal movement, without any uterine contractions. It serves as a surrogate measure of the developing fetal autonomic nervous system and adequacy of uteroplacental function.
- It is more specific than sensitive - a better indicator of fetal health than fetal illness
- Has been in use for over 40 years for second and third trimester fetal well-being assessment
Principle Behind NST
FHR monitoring relies on the coupling of fetal neurologic status to cardiovascular reflex responses. Fetal movement normally triggers FHR acceleration. The absence of this coupling may indicate fetal compromise.
Criteria for Reactive NST (Normal/Reassuring)
A reactive NST is defined as:
At least 2 FHR accelerations of ≥15 beats/min above baseline, each lasting ≥15 seconds, within a 20-minute period
| Gestational Age | Criteria |
|---|---|
| ≥32 weeks | Accelerations ≥15 bpm for ≥15 seconds |
| <32 weeks | Accelerations ≥10 bpm for ≥10 seconds (modified criteria, as younger fetuses have smaller accelerations) |
How the Test is Performed
- Patient is connected to an external cardiotocograph (CTG)
- Monitoring window: 20 minutes initially
- If non-reactive at 20 min: attempt to arouse the fetus (vibroacoustic stimulation)
- Fetal sleep cycles can last 30-40 minutes - the NST may take up to 60-100 minutes to complete if initially non-reactive
- A term fetus rarely goes more than 60 minutes (certainly not >100 minutes) without meeting reactivity criteria
Interpretation
| Result | Definition | Action |
|---|---|---|
| Reactive (Normal) | ≥2 accelerations of ≥15 bpm × ≥15 sec in 20 min | Reassuring; repeat as per schedule |
| Nonreactive | Criteria not met after adequate observation | Further evaluation needed (BPP or CST) |
Important: A nonreactive NST does not indicate fetal jeopardy by itself - it is an indication for further evaluation, not immediate intervention.
False-Negative Rate
- The false-negative rate (fetal death within 1 week of a reactive NST) is approximately 1.9 per 1000 - Creasy & Resnik's Maternal-Fetal Medicine
Causes of Non-Reactive NST (Not All Pathological)
- Fetal sleep state (normal behavioral state - most common cause)
- Extreme prematurity
- Maternal narcotics or magnesium sulfate
- Fetal cardiac or CNS anomalies
- True fetal compromise/hypoxia
Indications for NST
Conditions associated with increased fetal morbidity/mortality, including:
- Gestational diabetes mellitus (GDM)
- Chronic hypertension
- Pre-eclampsia
- Post-term pregnancy
- Intrauterine growth restriction (IUGR/FGR)
- Decreased fetal movements
General protocol:
- Weekly NST from 32 weeks for low-to-moderate risk pregnancies
- Earlier and more frequent testing for high-risk pregnancies (individualized)
NST in Context: Comparison with Other Surveillance Methods
| Test | What it Measures | Notes |
|---|---|---|
| NST | FHR accelerations with fetal movement | Primary screening tool; no contraindications |
| CST (Contraction Stress Test) | FHR response to uterine contractions | More specific but contraindicated in preterm labor, placenta previa |
| BPP (Biophysical Profile) | 5 parameters (NST + 4 USS features) | More sensitive; score 8-10 reassuring, ≤4 ominous |
| Modified BPP | NST + Amniotic Fluid Index (AFI) | As effective as full BPP; identifies SGA fetuses |
NST as Part of Biophysical Profile (BPP)
The NST is 1 of 5 components of the BPP:
| BPP Component | Normal (2 pts) | Abnormal (0 pts) |
|---|---|---|
| NST | ≥2 accelerations ≥15 bpm for ≥15 sec | <2 adequate accelerations |
| Amniotic fluid volume | ≥1 pocket ≥2×2 cm | No 2×2 cm pocket or AFI <5 |
| Fetal breathing movements | Sustained FBM ≥30 sec | <30 sec of FBM |
| Fetal body movements | ≥3 limb/gross body movements | <3 movements |
| Fetal tone | Extremities in flexion + ≥1 extension | Extension at rest / no return to flexion |
Total BPP Score: 8-10 = reassuring | 6 = suspicious (further evaluation) | ≤4 = ominous (immediate intervention)
Advantages of NST
- No contraindications
- Easily reproducible
- Relatively inexpensive
- Well accepted by patients
- In the 30-minute BPP window, only 10-12% of third-trimester fetuses are nonreactive; this drops below 6% by 40 minutes
Sources: Textbook of Family Medicine 9e, p. 500-501; Creasy & Resnik's Maternal-Fetal Medicine, p. 718
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