Describe non-stress test — its aetiology, clinical features and management.

Definition

The non-stress test (NST) is a non-invasive test of fetal wellbeing that records the fetal heart rate in relation to fetal movements. Acceleration of the heart rate with movement reflects an intact fetal autonomic nervous system and adequate oxygenation.

Method

With the mother semi-reclining (left lateral tilt), the fetal heart rate and movements are recorded by cardiotocography for 20–40 minutes; the mother marks each fetal movement.

Interpretation

• Reactive (reassuring) — at least two accelerations of ≥ 15 beats/min lasting ≥ 15 seconds within a 20-minute period (≥ 10 bpm for 10 s before 32 weeks).
• Non-reactive — accelerations absent; needs prolongation of the test, vibroacoustic stimulation, or further evaluation by biophysical profile or contraction stress test.

Significance and Limitations

A reactive trace is strongly reassuring of fetal wellbeing for the next several days; it is widely used in high-risk pregnancies (IUGR, diabetes, hypertension, post-term, reduced fetal movements). A non-reactive trace has a high false-positive rate (a sleeping fetus, sedation or prematurity), so it must be interpreted with the clinical picture and confirmed by additional testing.

Related Tests

• Vibroacoustic stimulation — a sound stimulus to rouse a sleeping fetus and shorten a non-reactive trace.
• Contraction stress test — the fetal-heart response to induced uterine contractions; late decelerations (a positive test) suggest uteroplacental insufficiency.
• Modified biophysical profile — the NST combined with the amniotic-fluid index.

Indications

Antepartum surveillance in high-risk pregnancies — hypertension/pre-eclampsia, diabetes, intra-uterine growth restriction, post-term pregnancy, Rh isoimmunisation, reduced fetal movements and multiple pregnancy.

A Note on WHY Accelerations Mean a Healthy Fetus

A useful concept is understanding why the presence of accelerations of the fetal heart is so reassuring — the entire basis of the non-stress test. The test rests on a simple physiological chain: when a healthy fetus moves, its sympathetic nervous system produces a transient acceleration of the heart rate. For this to occur, the fetus must have an intact and functioning central nervous system, intact autonomic control of the heart and — crucially — adequate oxygenation, because a hypoxic or acidotic fetus becomes inactive and its cardiac reactivity is blunted. An acceleration is thus a small demonstration that the whole system is working. Hence a 'reactive' trace — two or more accelerations of ≥15 beats per minute lasting ≥15 seconds within 20 minutes, on a normal baseline with normal variability — has a very high negative predictive value: significant fetal hypoxia is most unlikely in the following week. A non-reactive trace, however, is far less specific — it may simply mean the fetus is asleep (hence vibroacoustic stimulation, or extending the test to 40 minutes), or that it is premature, or that the mother has taken sedatives — so it prompts further testing (BPP, Doppler) rather than immediate delivery. Understanding this asymmetry explains exactly how the NST is used.

A reactive NST reflects an intact fetal autonomic nervous system.

• NST: a non-invasive recording of the fetal heart rate with fetal movements, used for antepartum fetal surveillance.
• Reactive (reassuring) = ≥2 accelerations of ≥15 bpm lasting ≥15 s in 20 minutes, with a normal baseline (110–160) and variability.
• Accelerations require an intact, well-oxygenated CNS and autonomic system — hence their high negative predictive value.
• Non-reactive is non-specific (fetal sleep, prematurity, sedation) → extend to 40 min, use vibroacoustic stimulation, then BPP/Doppler.
• Decelerations (especially late) with reduced variability suggest fetal compromise → further assessment or delivery.