What Is a Non-Stress Test?

Monitoring to Ensure Fetal Health

A prenatal non-stress test (NST) is a common test done before birth (prenatal). It is used to ensure the health of the fetus before labor. The test assesses fetal heart rate and movement at around 26 to 28 weeks of gestation, but can also be done later in the pregnancy as needed.

"Non-stress" means that there is no stress put on the fetus (for example, an attempt to get the heart rate up) during the test. This type of test is usually done when the fetus is considered high-risk. 

A non-stress test is noninvasive and does not pose any type of risk to the pregnant person or the fetus. If the test is failed, it usually indicates that more tests, further monitoring, or special care orders will be needed.

nonstress test

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Non-Stress Test Purpose

There are many reasons a non-stress test is performed when a person is pregnant. Overall, the test helps to evaluate the health of the fetus, providing useful information about the oxygen supply to the fetus, which is linked with fetal movement.

Later in pregnancy, the fetal heart rate typically increases with the fetus's physical activity. If fetal hypoxia (or other conditions) is present, the response can be disrupted because there is a lack of adequate oxygen. 

A fetal non-stress test can also be done for other reasons. A provider might do a non-stress test if:

  • The fetus is at risk (secondary to lack of adequate oxygenation)
  • Newborn complications have occurred (usually in high-risk pregnancies)
  • The pregnant person has had stillbirths in the past

These situations are often a result of high-risk pregnancies or fetal hypoxemia (lack of adequate oxygenation).

Conditions or situations that might warrant a fetal non-stress test include:

  • A multiple pregnancy with complications
  • Certain medical conditions during pregnancy (such as type 1 diabetes, high blood pressure, or heart disease)
  • Past-due pregnancy (two weeks or more past the due date)
  • A history of complications during pregnancy (such as gestational diabetes)
  • Decreased fetal movement
  • RH sensitization (a serious condition of pregnancy involving sensitization that occurs when the pregnant person’s blood is RH negative and the fetus is RH negative)
  • Preeclampsia (a condition during pregnancy involving high blood pressure and high protein in the urine)
  • Fetal growth problems
  • Low amniotic fluid (oligohydramnios)
  • Other conditions including drug use in the pregnant person, kidney insufficiency, liver disease, and others

The non-stress test can also be part of ongoing monitoring and testing or done during delivery. A provider may order the test at certain intervals (such as once a week, or even daily) depending on the health of the fetus.

The test is considered low risk for the pregnant person and the fetus, so it can be conducted as frequently as needed. However, it does not have any diagnostic value other than identifying whether fetal hypoxia occurs during the test.

Risk Factors

The fetal non-stress test does not pose any physical risk for the fetus or the pregnant person, but if it shows fetal hypoxia has occurred, parents may become anxious. 

It's also important to know that even when the results of the test are negative (no hypoxia is detected) it does not have additional diagnostic value. Therefore, other conditions or problems might be present.

What to Expect During the Test

How the Non-Stress Test Is Performed

Non-stress tests are typically started after week 28 of pregnancy and done as often as needed up to delivery. 

There is no specific preparation required for a non-stress test. It is usually done during an outpatient office or outpatient hospital visit.

  1. Before the non-stress test begins, the pregnant person’s blood pressure is taken.
  2. During the procedure, the pregnant person sits in a reclined position and their blood pressure is taken at regular intervals. 
  3. The fetal monitor has two sensors that are placed around the pregnant person’s abdomen. One sensor measures uterine contractions and the other keeps track of the fetal heart rate. A graph is displayed on a computer screen showing the information being tracked during the test. 

The entire test usually takes around 20 to 30 minutes, but it can take longer (up to an hour). If the fetus is inactive, the test time might need to be extended. A noise-making device can be placed on the pregnant person’s abdomen to stimulate the fetus when it is inactive. 

Test Results

A nurse will observe the fetal heart rate during the test to assess how the fetus's heart rate reacts when it moves (the heart rate should increase).

A reactive test shows an increase of at least 15 beats per minute (over the baseline heart rate) lasting at least 15 seconds, within a 20-minute time frame. The average baseline heart rate for a fetus is between 110 to 260 beats per minute.

A nonreactive test is when no acceleration of the fetal heart rate occurs (according to the parameters that must be met for a reactive test). This does not necessarily indicate that there is a problem with the fetus—it might just be sleeping during the test. However, a nonreactive non-stress test may indicate that more in-depth testing, such as a biophysical profile (BPP), needs to be done. 

A biophysical profile (BPP) is an ultrasound examination that monitors fetal muscle tone, fetal body movements, breathing movements, and fetal heart rate.

After the Procedure

Once the non-stress test has been completed, the healthcare provider will discuss the test results with you. You can bring up any questions or concerns that you have. When you are done, you will be able to go home. Your provider will let you know if any additional tests need to be scheduled.

Frequently Asked Questions

How long does a non-stress test take?

A fetal non-stress test usually takes around 20 to 30 minutes, but it can take up to an hour. It might take longer if the fetus is sleeping or not very active at the time the test is being done.

How do you wake a baby up during a non-stress test?

A noise-making device can be placed on the pregnant person’s abdomen to stimulate the fetus if they are not particularly active while the test is being done.

What does it mean when a fetus fails a non-stress test?

A nonreactive non-stress test can indicate that the fetus needs more monitoring and, sometimes, more involved testing.

That said, it's important to remember that a non-stress test determines only whether fetal hypoxia is present; it does not detect or diagnose any other conditions.

Therefore, passing the non-stress test is not the only "clean bill of health" a fetus and pregnant person need to ensure that the pregnancy is going smoothly. Keeping up with regular prenatal appointments will help ensure the health of the pregnancy.

 

A Word From Verywell

Having a fetal non-stress test is not stressful for the fetus, but it can be stressful for the pregnant person, who may be concerned that the results of the test might not be what is expected.

The test is safe for the fetus and the pregnant person and usually doesn't take very long. The test might take longer if the fetus is sleeping or not very active when the monitor is trying to measure its heart rate and movement.

Your provider will discuss the results of the test with you and make sure that your concerns are addressed and your questions are answered. If they would like you to have the test again, or want you to have additional tests, they will let you know.

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Article Sources
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