What is the NST?

By | 6 May 2021

What is the NST?

Following the completion of the 2nd month of pregnancy, the living beings in the uterus until the moment of birth are called a fetus. The only purpose of follow-up before birth is to maintain the vitality of the fetus in the womb.

When contractions occur in the uterus (womb) where the fetus is located, the amount of blood reaching the fetus in the mother’s womb decreases and the blood pressure of the fetus decreases. To balance this situation, the heartbeat of the fetus accelerates. The aim here is to pump the small amount of blood that reaches it faster and to ensure that the developing cells, tissues, and organs reach sufficient oxygen and nutrients within the contraction period.

Acceleration in the fetal heart can occur during movement as well as in adults.

In the follow-up of the baby in the womb, an evaluation method consisting of a combination of various parameters called ““fetal ‌ biophysical profile (‌BFP)” is used.

There are 5 components in this profile, 4 of them are based on the observations and calculations made by ultrasonography, and one is a method used to evaluate how the heartbeat of the fetus is affected by the movement of the fetus called the “non-stress test”.

While the positive and expected results get 2 points during the evaluation of these 5 parameters, if the expected findings cannot be obtained or negative findings are obtained, the result of the parameter is included in the calculation process as 0 points.

If the result of calculating the score with BFP is between 8-10 points, it is considered normal, 6 and 7 points as doubtful, and if it is less, it is considered as an abnormal result.

The respiratory movements of the fetus, body movements, tonus (passive movement of the muscles), and the amount of fluid in the sac in which it is contained constitute the 4 parameters observed during ultrasonography.

Fetal respiratory movements

The presence of regular respiratory movements of the baby in the mother’s womb, which occurs for half a minute or more within half an hour, is evaluated.

• ‌Fetal body movements

Detection of at least 3 different body or limb movements during the observation made within half an hour is considered normal.

• ‌Fetal ‌tonus

The hands are expected to open or close with the transition of at least one of the arms and legs of the baby in the mother’s womb from a tense ‌ (extension state) to a looser ‌ (flexion state) state.

Amniotic fluid amount

The fluid that makes up the environment in which the baby is in the mother’s womb is called the amniotic fluid. This liquid contributes to the protection of the baby and the development of its organs. An increase in the amount of fluid as the pregnancy progresses is a natural and expected situation.

In the measurements made with the help of ultrasonography in the bladder, the value measured for the deepest single pocket is greater than 2 centimeters, which is considered as the adequacy criterion of ‌amniotic fluid.

NST is a test that enables the recording of the heart sounds of the fetus in the womb with the help of an electronic monitor and to examine how the heart of the fetus responds to the movement of itself and its environment.

The NST is one of the tests most frequently applied by physicians to evaluate the well-being of the fetus before birth. The reason it is called “nonstress” is that it does not cause any stress on the fetus during the application of the test.

The working principle of “NST (non-stress test)” is based on the examination of the relationship between fetal heart rhythm and fetal movement. The fetal heart is expected to react to the movements. It is recommended to be careful, as the obstacle against the formation of this reflex response can occur in many different situations, from a deep sleep of the fetus to oxygen deprivation.

What are the special pregnancy situations that require the use of ‌‌the NST?

In many different situations and ailments, STNST can be applied to evaluate the well-being of the baby in the womb.

• In pregnancies where growth retardation is detected in the examinations of the baby in the mother’s womb

• Those with diabetes before pregnancy or in pregnancies where diabetes occurs and treated during pregnancy

• In pregnancies with preeclampsia or high blood pressure

• In pregnancies where fetal movements are found to be less

• In pregnancies past the normal delivery time

• In multiple pregnancies

• In recurrent pregnancy losses

• Pregnant women with systemic ‌lupus erythematosus and antiphospholipid antibody syndrome

• In pregnancies where amniotic fluid is detected less than normal

• In blood conflicts

How is ‌the NST done?

Nonstress test can be applied, if deemed appropriate by the physician, just before and at the beginning of the 7th month of pregnancy. If there is no need for the test, the 32nd week is generally preferred as the starting date. Since the relationship between the nervous system and the heart of the fetus is not fully developed before the 32nd week, findings different from normal are detected more frequently in the test results.

T‌‌, the NST contributes to the evaluation of whether high-risk pregnancies cause side effects such as lack of oxygen on the fetus and the fetal deaths that may occur in the mother’s womb.

The frequency of application is variable and takes place by the decision of the relevant physician. It can be re-applied every 1-2 weeks in high-risk pregnancies.

It should be kept in mind that the data obtained depends only on the events that occurred at the time of the test.

Using a monitor called a cardiotocograph (electronic fetal monitor), allows the mother’s heart rate and blood pressure to be examined in addition to the evaluation of the functioning of the heart of the fetus, what activity it follows, and contractions.

The ‌‌NST can be performed in a sitting position with the head slightly raised and sitting, or in a side-lying position. The electrical activity is monitored with the help of the transducer, which is placed outside the body and allows the energy reaching it to be transformed into another form of energy, allowing it to be recorded on the graph.

Feeling that the fetus is moving starts approximately after the 18th week of pregnancy. The values ​​for which the number of movements of the fetus is considered normal within a certain period of time is a minimum of 10 fetal movements within 180 minutes. When the mother feels that the fetus is moving, it presses the button to mark the moment of movement on the readings performed by the device.

With the initiation of the test, it is expected that the heart rate of the fetus will accelerate from the basal rate of 15 beats (acceleration), which lasts a minimum of 15 seconds and within 60 seconds. The basal rate refers to the average value of fetal heart rate over a 10-minute period.

The duration of the test is at least 20 minutes, and it can be applied for longer periods, depending on the physician’s decision, in cases such as waiting for the baby to be active.

How is the NST interpreted?

The results of ‌NST are generally classified and interpreted in 2 different ways.

• Reactive ‌NST

It is defined as the occurrence of at least 2 or more changes in the functioning of the heart of the fetus, defined as acceleration during a 20-minute test.

• ‌‌Non-Reactive ‌NST

If the expected changes have not occurred after starting the test, the test can be continued for at least another 20 minutes and an examination can be made whether the fetus is in a quiet sleep period or without oxygen (hypoxia).

One of the most common reasons for the test result to be non-reactive is that a normal baby is in a longer than normal sleep state.

To accelerate the realization of this distinction, a fetal stimulation technique called “vibroacoustic stimulation” can be tried by giving a short sound to the head of the fetus with the help of a tool placed on the belly of the expectant mother.

‌‌The non-reactive term means that the application time of the test exceeds 40 minutes and the expected ‌accelerations cannot be detected during this time.

‌‌ Apart from the fact that the fetus is not active during sleep or it may be receiving insufficient oxygen, non-reactive test results may also occur due to the mother’s smoking, the drugs used by the mother for various health conditions, or the abnormal heart and neurological structures of the fetus.


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