The Non-Stress Test (NST) is a common, non-invasive procedure used during the third trimester of pregnancy to evaluate a developing baby’s health. This assessment focuses on the baby’s heart rate patterns and movement, providing healthcare providers with information about fetal well-being. The primary purpose of the NST is to ensure the fetus is receiving sufficient oxygen, which measures their overall condition. Understanding the technical aspects of this test can help expectant parents feel more informed about their results.
Purpose and Mechanism of the Non-Stress Test
The non-stress test is named for the fact that it places no physical stress on the fetus during the procedure. It is generally performed in the third trimester, typically after 28 weeks of gestation, because this is when the fetal nervous system is mature enough for the heart rate to consistently respond to movement. The test is often ordered for pregnancies considered high-risk, such as those involving maternal diabetes, high blood pressure, or when a mother reports decreased fetal movement.
During the test, the pregnant person lies in a reclined position, often tilted slightly to the left, and two elastic belts are placed around the abdomen. One belt contains a sensor that uses Doppler technology to detect and record the fetal heart rate, while the other belt monitors for uterine contractions or fetal movement. The data is tracked on a monitor or printout, typically for a period of 20 to 30 minutes.
The core physiological principle of the NST is that a healthy, well-oxygenated fetus will naturally show an increase in heart rate when it moves. Just as an adult’s heart rate rises with physical activity, the baby’s heart rate should accelerate during periods of movement. If the fetus is experiencing suboptimal oxygenation, this specific heart rate response can become blunted or absent, indicating a need for further assessment.
Sometimes, the baby may be in a natural sleep cycle during the initial 20 minutes, which can cause the heart rate to remain steady or non-reactive. In these cases, the healthcare provider may attempt to rouse the baby, possibly by using an acoustic stimulator—a small buzzer placed on the abdomen—or by offering the mother a sugary drink. The test may be extended to 40 minutes to account for a full fetal sleep-wake cycle.
Interpreting Fetal Heart Rate Patterns
Reading the NST involves evaluating three primary components of the fetal heart rate tracing: the baseline rate, the presence of accelerations, and the absence of concerning decelerations. The Baseline Rate is the average heart rate of the fetus, measured in beats per minute (bpm), when the baby is not moving or experiencing a contraction. A normal baseline rate for a fetus is generally considered to be between 110 and 160 bpm.
The most important factor in determining a reassuring result is the occurrence of Accelerations, which are temporary increases in the fetal heart rate. The criteria for a test to be deemed “reactive” or reassuring are based on the baby’s gestational age. After 32 weeks of gestation, a reactive test must show two or more accelerations within a 20-minute window.
The standard for a healthy acceleration is often referred to as the “15×15 rule.” This means the heart rate must increase by at least 15 beats per minute above the baseline and last for a minimum of 15 seconds before returning to the baseline rate. For pregnancies under 32 weeks, the criteria are slightly less stringent, requiring a 10 bpm increase lasting 10 seconds. Meeting these criteria suggests the baby’s central nervous system and oxygenation status are functioning appropriately.
A Non-Reactive Test is one that does not meet the criteria for accelerations within a 40-minute monitoring period. A non-reactive result does not automatically mean the baby is in distress, as it can be caused by the baby sleeping, certain maternal medications, or maternal smoking. However, it does indicate the need for further investigation to rule out potential concerns about the baby’s oxygen supply.
The final element of the NST tracing is the presence or absence of Decelerations, which are drops in the fetal heart rate. While some types of decelerations, like mild variable decelerations, may not always be concerning, the appearance of Late Decelerations is considered a potentially serious finding. Late decelerations are characterized by the heart rate drop beginning after the peak of a contraction and returning to baseline after the contraction has ended, a pattern that can suggest placental insufficiency.
Clinical Management Following Results
When an NST result is determined to be reactive, it indicates a high likelihood of fetal well-being. A reactive test is associated with a low risk of fetal death in the subsequent week, confirming the baby’s oxygen supply is adequate. Monitoring typically continues as scheduled according to the ongoing care plan.
If the non-stress test is non-reactive, the healthcare team will move immediately to additional testing to gain a more complete picture of the baby’s health. The most common next step is a Biophysical Profile (BPP), which combines the non-stress test with a comprehensive ultrasound examination. This ultrasound assesses four additional parameters:
- Fetal breathing movements
- Gross body movements
- Fetal tone
- The amount of amniotic fluid (Amniotic Fluid Index or AFI)
Another potential follow-up is the Contraction Stress Test (CST), which evaluates how the baby’s heart rate reacts to uterine contractions, either spontaneous or induced. This test helps determine if the fetus can withstand the temporary reduction in blood flow that occurs during a contraction. The results from the BPP or CST, combined with the initial NST, guide the management plan, which could range from increased surveillance to delivery.