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Non-stress test: Procedure and its crucial importance during pregnancy

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Obstetrics Department
Understand what a Non-stress test (NST) is, its procedure, and its vital role in monitoring fetal health, helping expectant mothers feel more secure throughout their pregnancy.

A Non-stress test (NST) is a non-invasive, safe, and painless examination widely used during pregnancy to assess fetal well-being in the womb [1]. This technique helps in the early detection of abnormalities, ensuring a safe pregnancy and empowering expectant mothers to actively care for their own and their baby's health [1].

1. What is a Non-stress test (NST)?

1.1. What is a Non-stress test?

A Non-stress test is a fetal heart rate monitoring test (Cardiotocography - CTG) that records the fetal heart rate and uterine contractions when the baby moves or when there are uterine contractions [1, 2]. The name "non-stress" comes from the fact that this technique does not cause any pressure or risk to the fetus during the procedure [1].

1.2. Purpose of a Non-stress test

The main purpose of an NST is to assess the health status of the fetus before birth [1]. NST is based on the principle that in a healthy fetus, not deprived of oxygen or having neurological problems, the heart rate will increase with movement [3, 4]. Monitoring this response helps obstetricians assess the oxygen supply and cardiovascular function of the fetus. If NST results show an abnormal fetal heart rate, it may suggest that the fetus is not receiving enough oxygen, and further tests or medical interventions may be needed [1].

2. When is a Non-stress test performed? (Indications)

NST is typically performed in the third trimester of pregnancy, most commonly after 28 weeks [1]. However, the specific timing and frequency of NST will be individualized based on the mother's and baby's health status, as well as other risk factors [2].

You may need an NST if:

  • The fetus appears to move less than usual [1, 2].
  • The pregnancy is post-term (42 weeks or more) [1].
  • You have chronic medical conditions that make the pregnancy high-risk, such as high blood pressure, diabetes, heart disease, or coagulation disorders [1, 2].
  • You have had complications in a previous pregnancy [1].
  • You are Rh-negative and the fetus is Rh-positive (Rh incompatibility) [1].
  • You are carrying multiple fetuses (twins, triplets, or more) [1, 2].
  • Other conditions such as fetal growth restriction, polyhydramnios or oligohydramnios, or fetal anomalies [2].

2.2. Timing of Non-stress test

Experts generally recommend initiating fetal surveillance from 32 weeks of gestation for most cases, as by this time, NST typically yields a "reactive" result in most fetuses [2]. For high-risk pregnancies, surveillance may begin earlier depending on the physician's assessment [2].

3. Non-stress test procedure

NST is a simple, safe, and painless procedure for both mother and baby [1].

3.1. Preparation before the test

You do not need any special preparation for an NST [1]. However, your doctor may ask you to urinate before the test to empty your bladder [1]. In some cases, you may be advised to eat a light snack or drink something sweet before the test to encourage fetal movement [1].

3.2. Steps for performing a Non-stress test

The NST procedure typically takes place in a clinic or the hospital's prenatal area and includes the following steps [1]:

  1. Lying position: You will be asked to lie on your back in a recliner or examination table, usually tilted to one side (often the left) or in a semi-Fowler's position to avoid vena cava compression [2, 5].
  2. Placement of equipment: A special gel will be applied to your abdomen. Then, two belt-like devices will be placed around your abdomen. One device will measure the fetal heart rate (using a Doppler ultrasound transducer), and the other will record uterine contractions [1, 2].
  3. Monitoring and recording: The doctor or technician will move the transducer over your abdomen until the fetal heart rate is found. The heart rate and contractions will be displayed on a screen or printed on paper [1].
  4. Recording movements: You may be asked to press a button on the device each time you feel the fetus move. This helps the doctor note the correlation between fetal movement and heart rate acceleration [1].
  5. Fetal stimulation (if needed): If the fetus is inactive or sleeping during the test (usually lasting about 20-30 minutes), the doctor may use a small vibroacoustic stimulator placed on your abdomen to wake the baby. This is harmless to the fetus [1, 2].

3.3. Duration of the test

An NST usually lasts about 20 to 30 minutes [1]. However, if the fetus is sleeping or less active, the process may extend up to 40 minutes to wait for the baby's wake-sleep cycles [2].

4. Interpretation of Non-stress test results

NST results are primarily classified as "reactive" or "non-reactive" [1].

4.1. Reactive result

This is a normal and reliable result, indicating a healthy fetus [1]. An NST is considered reactive when the fetal heart rate increases by at least 15 beats per minute above the baseline and lasts for at least 15 seconds, occurring at least twice within a 20-minute period [1, 6]. For fetuses under 32 weeks, this criterion may be slightly modified, with an increase in heart rate of at least 10 beats per minute for at least 10 seconds [2].

4.2. Non-reactive result

A non-reactive result means that the fetal heart rate does not accelerate sufficiently or does not have enough accelerations to meet the "reactive" criteria within the monitoring period [1]. This result does not necessarily mean that the fetus has a health problem. The fetus may be sleeping, or the mother may be taking certain medications that can affect fetal activity [1, 2]. However, if the result is non-reactive, the doctor will order additional tests such as ultrasound, Biophysical Profile (BPP), or Contraction Stress Test (CST) to further assess the fetal condition [1].

4.3. Factors that can affect results

Several factors can make an NST non-reactive or difficult to interpret accurately, including:

  • Fetal sleep cycles: Fetuses have sleep-wake cycles. If the test is performed when the baby is in deep sleep, the heart rate may show fewer changes [1].
  • Medications: Some medications the mother is taking (e.g., sedatives, antihypertensive drugs) can reduce fetal activity and affect the results [2].
  • Gestational age: Preterm fetuses (before 32 weeks) often have non-reactive NSTs due to their immature nervous system [2].
  • Mother's health status: Maternal fever or other medical conditions can also affect the results [2].

5. Benefits and limitations of Non-stress test

5.1. Key benefits

  • Non-invasive and safe: NST is an absolutely safe method for both mother and baby, causing no pain or risk [1].
  • Early risk detection: Helps in the early detection of signs of fetal distress or problems with oxygen supply to the fetus, allowing for timely intervention [1, 3].
  • Reliable: A "reactive" NST result is generally very reliable in identifying a healthy fetus [2].
  • Simple, easy to perform: The procedure is relatively simple and quick [1].

5.2. Limitations and risks

  • False positive rate: A "non-reactive" result does not always indicate a serious fetal health problem; it could be due to the fetus sleeping or other factors [1]. This can lead to unnecessary additional tests, causing anxiety for expectant mothers and increasing healthcare costs [2].
  • Cannot predict acute events: NST is less effective at predicting sudden acute events such as placental abruption or cord prolapse [2].
  • Not a standalone method: NST is often combined with other monitoring methods such as ultrasound, BPP to get a more comprehensive view of fetal health [1].

6. Frequently Asked Questions (FAQ)

  1. Is a Non-stress test painful? A Non-stress test is completely painless for both mother and baby. You will only feel the belts around your abdomen and perhaps a slight pressure from the ultrasound transducer.
  2. Do I need any special preparation before an NST? No special preparation is usually needed. However, you should urinate before the test. Your doctor may advise you to eat a light snack or drink something sweet beforehand to encourage fetal activity.
  3. Is a non-reactive NST result concerning? A non-reactive result is not always a sign of danger. The fetus may be sleeping or affected by medication. Your doctor will further evaluate and may request other tests to determine the cause.
  4. Can a Non-stress test detect all fetal problems? NST is an important screening tool but cannot detect all fetal problems. It focuses on assessing fetal heart rate and movement to identify signs of fetal distress related to oxygen deprivation.
  5. How often should I have an NST? The frequency of NST depends on your and your fetus's health status. For high-risk pregnancies, your doctor may recommend weekly or twice-weekly tests. For normal pregnancies, NST is only performed when specifically indicated.

7. Information Review & References

Information Review:

  • Professional Consultation: CN. Truc Thanh Mai - Department of Obstetrics and Gynecology, University Medical Center HCMC
  • Date of Update: 2025-11-20

References:

The information above is for reference purposes only and should not be considered a recommendation. Please consult a doctor for detailed advice.

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