NST (Non-Stress Test)

What Is the NST (Non-Stress Test)?

The NST, or Non-Stress Test, is a non-invasive and safe fetal monitoring method that simultaneously records the baby’s heart rate and its response to movements during the third trimester of pregnancy. What makes the test “non-stress” is that no external stress is applied to the fetus; only how the heart rate responds to the baby’s own spontaneous movements is observed. In Ankara, Obstetrics and Gynecology Specialist Prof. Dr. Melahat Atasever applies the NST as an integral part of the routine monitoring protocol in the follow-up of high-risk pregnancies.

The NST is based on the simultaneous recording of the fetal heart rate (FHR) and uterine activity with a cardiotocography (CTG) device. In a healthy, well-oxygenated fetus, the heart rate increases by a certain amount during the baby’s movements; in medical terminology, this increase is called an “acceleration.” The presence and adequacy of accelerations is a strong indicator that the baby’s autonomic nervous system is functioning healthily and that oxygenation is sufficient.

The test concludes in one of two ways: “reactive” or “non-reactive.” A reactive NST means that a sufficient number and height of accelerations were observed within the defined period, and it reflects the baby’s healthy condition. A non-reactive NST is the situation in which the expected accelerations are not seen; however, this does not always mean there is a problem, and additional evaluations such as a stress test or a biophysical profile come into play.

The NST is completely non-invasive, involves no radiation, and carries no risk for mother or baby. It is the most frequently used test of fetal well-being, particularly in the final period of pregnancy monitoring.

Why and When Is the NST Performed?

The main purpose of the NST is to indirectly assess the fetus’s oxygenation and overall well-being. To understand whether the placenta is supplying the baby with sufficient oxygen, heart rate variability and accelerations are examined. In this way, potential problems can be detected before they cause symptoms.

Indications for the NST include gestational diabetes, preeclampsia, chronic hypertension, intrauterine growth restriction (IUGR), multiple pregnancy, oligohydramnios (low amniotic fluid), post-term pregnancy (beyond 42 weeks), autoimmune diseases such as antiphospholipid syndrome, decreased fetal movement, and placenta previa. The mother reporting that the baby is moving less is, on its own, sufficient reason for an NST.

The NST is generally performed from the 28th week of pregnancy onward. Before this week, the fetal autonomic nervous system is not sufficiently mature, so the acceleration response cannot be reliably assessed. In normal, low-risk pregnancies, a weekly NST after week 36 is sufficient. In high-risk pregnancies, the NST can be performed twice a week or more frequently from week 28 onward.

Eating before the procedure is advised. The physiological rise in blood sugar increases fetal activity, which raises the likelihood of obtaining a reactive NST. In NSTs performed on an empty stomach, the baby may be in a sleep period, which can produce a misleading non-reactive result.

How Is the NST Performed? The Step-by-Step Process

The expectant mother is placed in the NST room in a left lateral or semi-sitting position. Prolonged lying flat on the back is avoided; the uterus can press on the large vessels and negatively affect fetoplacental circulation. After conductive gel is applied to the abdomen, two sensors are secured with elastic belts.

The fetal heart rate sensor (cardiograph) is placed over the area where the baby’s heart sounds are heard most clearly, and the toco sensor is placed on the upper part of the uterus. Once the device begins recording, the mother is given a hand-held button and asked to press it each time she feels the baby move. The standard NST duration is 20-40 minutes. If the baby is in a sleep period, the duration can be extended or vibroacoustic stimulation (VAS) can be applied.

All data are recorded on paper or a digital screen as the NST tracing. Prof. Dr. Melahat Atasever evaluates the tracing comprehensively in terms of the number and height of accelerations, the baseline heart rate, short- and long-term variability, and the presence of decelerations.

What Should the Baby’s Heart Rate Be on the NST?

In a healthy fetus, the baseline fetal heart rate should be between 110 and 160 beats per minute. When the baseline rate falls outside these limits, tachycardia (>160 beats/min) or bradycardia (<110 beats/min) is considered; both situations require clinical evaluation.

The standard criterion for a reactive NST is as follows: at least two accelerations must be observed within 20 minutes. Each acceleration must be at least 15 beats/min above the baseline and last at least 15 seconds. Before the 32nd week of pregnancy, these criteria are set somewhat lower; an increase of 10 beats/min and a duration of 10 seconds are considered sufficient.

What Do the NST Contraction Values Mean?

The tocodynamometer records contraction activity. No significant contractions are expected during an NST; if a contraction does occur, its relationship to the heart rate is evaluated. Late decelerations occurring simultaneously with contractions are an important finding pointing to placental insufficiency and directly change clinical management.

Early or variable decelerations that do not occur in response to contractions are conditions arising from cord compression or head compression, and they are evaluated in context. In the presence of spontaneous contractions, the NST also takes on the character of a simple contraction stress test (CST).

How Are NST Results Interpreted?

A reactive NST refers to the presence of two or more adequate accelerations within 20 minutes and is strong evidence of the baby’s healthy well-being. After a reactive NST, the follow-up frequency is determined according to the clinical picture.

A non-reactive NST means that sufficient accelerations could not be obtained even after 40 minutes of monitoring. This is not necessarily a problem and may reflect a fetal sleep period. In this situation, vibroacoustic stimulation is applied, or further evaluation is performed with a biophysical profile (BPP). If the BPP score is low or the Doppler findings are abnormal, a decision for early delivery may come into consideration.

The Difference Between the NST and the Biophysical Profile (BPP)

While the NST evaluates only the fetal heart rate and movement, the BPP additionally covers fetal breathing movements, body movements, muscle tone, and the amount of amniotic fluid via ultrasound. The BPP is scored out of 10; a score of 6 or below is considered the critical threshold. In the presence of a non-reactive NST, the BPP is frequently requested as a complementary test because of its contribution to the diagnosis.

Prof. Dr. Melahat Atasever does not evaluate NST results merely as reactive or non-reactive; she interprets them together with the mother’s overall clinical condition and other follow-up tests (ultrasound, Doppler) to reach a decision appropriate to the clinical picture regarding whether the baby can safely continue to remain in the womb.