Non-Stress Test

The non-stress test is a cardiotocographic recording of the foetal heart rate without uterine stimulation, used to assess foetal wellbeing. In a well oxygenated, neurologically intact fetus, heart rate accelerates with movement. Reactivity is therefore a marker of intact autonomic function, and a reactive trace is strongly reassuring in the short term.

It is used in the third trimester in pregnancies at risk of placental insufficiency and in the assessment of reduced foetal movements.

Interpretation depends on the foetal sleep cycle. A non-reactive trace is often a sleeping fetus, which makes extended recording or stimulation necessary before any decision.

Indications. Reduced foetal movements, foetal growth restriction, hypertensive disorders, diabetes, post-term pregnancy, previous stillbirth, multiple pregnancy, and decreased amniotic fluid.

Preparation and technique. The mother sits or lies in a semi-recumbent or lateral position. The trace is recorded for 20 minutes and extended up to 40 minutes if it is non-reactive. A vibroacoustic stimulus can be used to wake a sleeping fetus. Frequency in high risk pregnancy is often twice weekly, and the schedule depends on the indication.

Contraindications and precautions. Antenatal testing is not used before viability. Maternal sedatives, opioids and magnesium sulphate lower reactivity.

Interpretation.

• Reactive: at least 2 accelerations of 15 beats/min lasting at least 15 seconds within a 20-minute period, at 32 weeks or later. Before 32 weeks the thresholds are 10 beats/min for 10 seconds.
• Non-reactive: no such accelerations during 40 minutes. This requires further assessment.
• Decelerations during the trace are interpreted by their shape, depth and timing relative to foetal movement, and persistent late or severe variable decelerations are abnormal even if accelerations are present.

Causes of a non-reactive trace. Foetal sleep, foetal hypoxia, foetal acidaemia, congenital anomalies, foetal infection, maternal medicines, and extreme prematurity.

Critical values and findings needing immediate action.

• A non-reactive trace with absent variability or repeated decelerations requires same day assessment with a biophysical profile or delivery planning.
• A non-reactive trace in a woman reporting absent foetal movements requires urgent ultrasound.

Limitations and artefacts. The false positive rate (non-reactive trace in a healthy fetus) is high, mostly from sleep. The negative predictive value for foetal death within a week is high. The test says little about growth or placental function over longer periods.

Follow up tests. Biophysical profile, amniotic fluid index, umbilical artery Doppler, contraction stress test, and growth ultrasound.

Special groups. Preterm fetuses are less likely to show accelerations and use lower thresholds. Diabetic pregnancies need assessment of growth and amniotic fluid alongside the test.