NST (Non-Stress Test) in Pregnancy: Why It's Done, How It Works and How to Read the Result

A non-stress test (NST) is a simple, painless test that records your baby's heart rate, usually for 20 to 40 minutes, while you rest and the baby moves. Healthy babies' heart rates speed up when they move, just as yours does when you walk. A "reactive" result, meaning the heart rate rose enough at least twice in 20 minutes, is reassuring. A "non-reactive" result isn't automatically a problem. The baby may be asleep, and your doctor will extend the test, wake the baby gently or arrange further checks such as an ultrasound or Doppler. It is called "non-stress" because nothing is done to stress the baby: no contractions are induced.

This guide explains who needs an NST and when, how to prepare, what happens during the test, how to understand the terms on your report, why a result may be non-reactive and what the next steps are.

What the NST measures

The NST uses a cardiotocograph (CTG) monitor, with two belts around your abdomen:

• One has an ultrasound sensor that picks up your baby's heartbeat.
• The other (the tocodynamometer) detects uterine tightening.

The monitor prints a trace showing the baby's heart rate (usually the upper line) and your uterine activity (the lower line). You may also press a button whenever you feel the baby move, which marks the trace.

The principle is that a baby with a healthy supply of oxygen and a healthy nervous system will show heart rate accelerations (temporary rises) with fetal movement. A baby who is not getting enough oxygen may lose that responsiveness.

When is an NST advised?

Usually after 28 weeks (often from 32 weeks), in pregnancies where there is a higher risk of problems or a concern. Common reasons:

• Reduced baby movements
• High blood pressure or pre-eclampsia
• Diabetes, including gestational diabetes on treatment
• Pregnancy beyond the due date (past 40 or 41 weeks)
• Poor fetal growth or low amniotic fluid
• Multiple pregnancy with complications
• Previous stillbirth
• Rh sensitisation or other blood group issues
• Chronic conditions such as kidney disease, thyroid disease, lupus, heart disease or asthma needing treatment
• Bleeding in pregnancy, or problems with the placenta
• Decreased amniotic fluid or abnormal Doppler
• Advanced maternal age in some centres
• IVF pregnancies in some centres, when risk factors are present
• A concern raised during routine checks

It may be repeated once or twice a week in some conditions. It is not done routinely in every healthy pregnancy.

How to prepare

• Eat a light meal or snack an hour or two before. Babies are often more active after you've eaten, and low blood sugar may reduce movements.
• Empty your bladder before the test.
• Wear comfortable clothes that give easy access to your abdomen.
• Avoid smoking beforehand, which can reduce baby movements.
• Take your medicines as usual unless told otherwise. Tell the staff about any.
• Bring your antenatal file and previous reports.
• Allow about an hour for the visit.

If your baby has been sleeping, a cold drink or a short walk can encourage movement. Don't skip the test because you think the baby is inactive: it is useful in that situation too.

What happens during the test

1. You sit in a reclining chair or lie semi-reclined, tilted slightly to the left. Lying flat on your back should be avoided, because the uterus can compress the large blood vessels and lower blood flow.
2. The staff place the two belts on your abdomen, with ultrasound jelly under the heart rate sensor.
3. You may be given a button to press at each fetal movement.
4. The tracing is recorded for about 20 minutes, extended to 40 minutes if needed.
5. Your blood pressure may be checked.
6. The staff review the trace and tell you the result.

It's not painful, and the baby is not affected by the monitor. You may hear the heartbeat as a rhythmic sound.

What the report shows

Baseline fetal heart rate

The average heart rate between accelerations and contractions. Normal is 110 to 160 beats per minute (bpm). A rate below 110 or above 160 for 10 minutes or more may need attention.

Variability

The beat-to-beat fluctuation of the heart rate, which reflects a healthy nervous system. Normal variability is 6 to 25 bpm. Reduced variability can occur when the baby is sleeping (usually for up to 20 to 40 minutes) or when it is under stress.

Accelerations

Brief rises in heart rate above the baseline. At or after 32 weeks, an acceleration is a rise of at least 15 bpm lasting at least 15 seconds. Before 32 weeks, the criteria are a rise of at least 10 bpm for at least 10 seconds.

Decelerations

Temporary drops in heart rate. Occasional, brief variable decelerations can be normal. Repeated or prolonged decelerations, or those after contractions, need attention.

Fetal movements

The movements marked by you or detected by the machine.

Contractions

Mild, irregular tightening is common. Frequent contractions may be noted.

Interpreting the result

| Result | Criteria (commonly used) | Meaning | |---|---|---| | Reactive | Normal baseline, normal variability and at least two accelerations in 20 minutes | Reassuring | | Non-reactive | No or fewer than two accelerations in 40 minutes | Needs further assessment; not necessarily abnormal | | Unsatisfactory | Poor tracing | Repeat the test |

Reactive

The test is reassuring: your baby's oxygen supply and nervous system appear to be working well at that moment. Your doctor will tell you when the next test is due. A reactive NST is associated with a very low chance of stillbirth within the following week, but it doesn't predict the future, so continue to monitor movements daily.

Non-reactive

A non-reactive trace doesn't always mean the baby is unwell. Common benign reasons:

• The baby is in a sleep cycle (lasting 20 to 40 minutes)
• Gestational age under 28 to 32 weeks, when babies are less reactive
• Medicines that affect fetal activity, such as sedatives, some blood pressure drugs like beta blockers, and opioids
• Maternal smoking, low blood sugar or being very anxious

Possible concerning reasons include reduced oxygen supply to the baby because of placental problems, low amniotic fluid, or other complications.

What happens next with a non-reactive NST?

• The test is extended for another 20 minutes, or repeated after a short break.
• The staff may gently stimulate the baby by a sound (vibroacoustic stimulation) or ask you to drink something sweet or change position.
• A biophysical profile (BPP) is often done: an ultrasound that scores breathing movements, body movements, tone, amniotic fluid volume, and combines it with the NST.
• A Doppler ultrasound may check blood flow in the umbilical artery and other vessels.
• If concerns continue, a contraction stress test (a monitored test with contractions) or admission for closer observation may be considered.
• In some cases, early delivery is advised if the baby shows signs of distress, particularly at or near term.

Your doctor will explain the reasons carefully.

Risks and limitations

• The NST is safe for you and your baby, with no radiation.
• It has a high false-positive rate: many non-reactive tests turn out to have a healthy baby. That's why follow-up tests are used and decisions are not made on one trace alone.
• It reflects the baby's condition at the time of the test only.
• It does not replace daily movement counting.

Counting baby movements: a test you can do at home

From about 28 weeks, get to know your baby's pattern. One common method is "count to 10": choose a time when the baby is usually active (after a meal), lie on your left side, and count how long it takes to feel 10 movements. Most babies manage this within about 2 hours, often much faster. Contact your doctor or hospital the same day if:

• The baby's movements are noticeably less than usual
• You don't feel 10 movements within 2 hours
• The baby's pattern has changed significantly

Don't wait until the next day, and don't use home dopplers or "wait and see" to reassure yourself. A sudden change in movements always needs assessment.

Frequently asked questions

What is an NST test in pregnancy?

A non-stress test that records your baby's heart rate with the baby's movements and your contractions, to check that the baby is doing well.

Is the NST harmful or painful?

No. It's painless and safe for you and your baby.

What does reactive NST mean?

The baby's heart rate rose adequately (at least twice in 20 minutes) with movements, which is reassuring.

What does non-reactive NST mean?

The heart rate didn't show enough accelerations during the test period. Often the baby is asleep, but further tests are done to be sure.

How long does an NST take?

Usually 20 to 40 minutes, sometimes longer.

How often should an NST be done?

It depends on your condition. For some high-risk pregnancies, once or twice a week after 32 weeks.

Does a normal NST mean my baby will be fine?

It's reassuring at the time of the test but isn't a guarantee. Keep monitoring movements and attending visits.

When to see a doctor

Contact your doctor or hospital immediately for reduced fetal movements, bleeding, leaking fluid, severe headache, vision changes, swelling or abdominal pain. Don't wait for your next scheduled test.

Care at Prakash Hospital Noida

Whether you live in Sector 18, Sector 62, Greater Noida West, or anywhere nearby, Prakash Hospital Noida is a trusted name for high-risk pregnancy care and fetal monitoring in Noida. Our obstetric team monitors your baby carefully and explains each result clearly.

To book a consultation, call the number.

Sources: American College of Obstetricians and Gynecologists practice bulletin on antepartum fetal surveillance; Royal College of Obstetricians and Gynaecologists guidance on reduced fetal movements. Criteria and schedules vary with local protocol.

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