The abbreviation NST stands for Non-Stress Test, which is a common prenatal screening procedure used to assess the health of a developing fetus. This test is non-invasive and painless, and it earned its name because it introduces no external stressors. The primary goal of an NST is to monitor the fetal heart rate and its response to the baby’s own movements, providing insight into the functionality of the central nervous system and overall well-being. A healthy fetus will typically show a temporary increase in heart rate when it moves, a sign that the baby is receiving sufficient oxygen from the placenta.
Why the Test is Performed
The Non-Stress Test is generally recommended for pregnancies considered high-risk or when there is a specific medical concern about the baby’s condition. The test is a method of antenatal surveillance, primarily performed after 28 weeks of gestation, when the fetal nervous system is mature enough to reliably show the desired heart rate changes.
Healthcare providers often order an NST if the mother reports a decrease in the usual frequency of fetal movements, which can sometimes be an early indicator of compromise. Certain maternal medical conditions also necessitate regular monitoring, such as pre-existing or gestational diabetes, high blood pressure (hypertension), or chronic kidney disease. Furthermore, the test is routinely used for pregnancies that have extended past the estimated due date or in cases where the baby is measuring small for its gestational age.
How the Non-Stress Test is Conducted
The Non-Stress Test is conducted in an outpatient setting and typically takes about 20 to 40 minutes to complete. The mother is usually positioned in a comfortable, semi-reclined chair or on an examination table, often leaning slightly to the left side. This positioning prevents pressure on a major blood vessel called the vena cava, ensuring optimal blood flow to the uterus and the fetus.
Two elastic belts are placed around the abdomen, each holding a specialized sensor against the skin. One sensor is a Doppler ultrasound transducer, which uses sound waves to detect and record the fetal heart rate on a continuous tracing. The second sensor, called a tocodynamometer, measures uterine contractions and records fetal movements felt by the mother.
The mother is given a handheld device and asked to press a button every time she feels the baby move, correlating the heart rate tracing with the observed activity. If the baby is inactive, the nurse may use a gentle method to encourage movement, such as a small, handheld vibratory device placed on the abdomen. This vibroacoustic stimulation helps to wake the fetus and elicit the necessary heart rate response for the test.
Understanding the Test Results
The results of a Non-Stress Test are categorized as “Reactive” or “Non-Reactive.” A Reactive result is considered normal and reassuring, indicating that the fetus is well-oxygenated and that its nervous system is functioning appropriately. To be deemed Reactive, the fetal heart rate tracing must show at least two accelerations over a 20-minute period.
An acceleration is defined as a temporary increase in the fetal heart rate above its baseline rate. For a fetus at 32 weeks gestation or greater, an acceleration must rise by at least 15 beats per minute and last for a minimum of 15 seconds. Before 32 weeks, the criteria are slightly less stringent, requiring an increase of 10 beats per minute lasting 10 seconds.
A Non-Reactive result occurs when the tracing fails to meet these specific criteria for accelerations within the 20-minute period. This outcome does not automatically mean the baby is in distress, as the fetus may simply be asleep, or certain medications the mother is taking may be temporarily affecting the baby’s activity. However, a Non-Reactive result warrants further investigation to rule out any potential issues with oxygen supply.
Next Steps Following an Unclear Result
A Non-Reactive NST result indicates the need for additional diagnostic testing to gain a more complete picture of fetal health. Clinicians often extend the monitoring period to 40 minutes or attempt to stimulate the fetus to confirm the non-reactive status. If the result remains non-reactive, the next step is typically to perform more definitive assessments.
The two most common follow-up tests are the Contraction Stress Test (CST) and the Biophysical Profile (BPP). The CST monitors the fetal heart rate response to mild uterine contractions to see how the baby tolerates a temporary reduction in blood flow. The BPP is a more comprehensive evaluation that uses ultrasound to look at five parameters:
- The NST result.
- Fetal breathing movements.
- Body movements.
- Muscle tone.
- The volume of amniotic fluid.
These secondary tests provide the necessary data to determine if intervention, such as early delivery, is required.