A non-stress test (NST) is a simple, non-invasive way to check how your baby is doing by monitoring their heart rate over time. It is one of the most commonly used tools for fetal wellbeing assessment in the third trimester.
What the Non-Stress Test Checks
The NST monitors fetal heart rate using a CTG (cardiotocography) machine — the same equipment used during labour. Two belt-mounted monitors are placed on your abdomen: one records the fetal heart rate and one records uterine activity. The test takes 20–40 minutes.
When an NST Is Ordered
- Reduced fetal movement or a kick count below 10 in 2 hours
- Postterm pregnancy (beyond 41 weeks)
- Gestational diabetes or pre-eclampsia
- SGA or IUGR on previous growth scan
- Multiple pregnancy requiring routine monitoring
- Maternal feeling of 'something is wrong'
What Reactive vs Non-Reactive Means
| Result | What It Means | Next Steps |
|---|---|---|
| Reactive | 2 or more heart rate accelerations of 15 bpm lasting 15 seconds in 20 minutes | Reassuring — usually no further action |
| Non-reactive | Fewer accelerations — but baby may be sleeping | Extend test to 40 minutes, provide juice, vibroacoustic stimulation |
| Non-reactive at 40 minutes | Requires further evaluation | Biophysical profile or Doppler scan |
Why Non-Reactive Doesn't Always Mean a Problem
The most common reason for a non-reactive result is that the baby is in a sleep cycle. Sleeping babies do not produce the heart rate accelerations that define a reactive test. Most non-reactive tests become reactive once the baby wakes — which is why waiting 40 minutes and sometimes using vibroacoustic stimulation (a buzzer on the abdomen) is standard practice.
Important
A genuinely non-reactive NST at 40 minutes requires further investigation — usually a biophysical profile (BPP) — to assess multiple aspects of fetal wellbeing before decisions about delivery are made.