The Non-Stress Test (NST) is a common, non-invasive procedure used during the third trimester of pregnancy to assess fetal well-being. This screening tool provides valuable information about fetal oxygenation and the functioning of the central nervous system. The test is named “non-stress” because it monitors the baby’s heart rate response to natural movement without inducing strain on the mother or fetus. The primary goal is determining if the fetus is receiving adequate oxygen supply and responding appropriately to its own activity.
What the Non-Stress Test Measures
The process involves placing two external sensor devices, called transducers, on the mother’s abdomen using elastic belts. One transducer uses Doppler ultrasound technology to monitor and record the Fetal Heart Rate (FHR), providing a continuous tracing of the baby’s heartbeat in beats per minute (bpm). The second transducer, known as a tocodynamometer or “toco,” detects and records uterine activity, including contractions or fetal movements.
These two distinct data streams are printed simultaneously onto a paper strip, resulting in two separate lines. The upper line displays the FHR, while the lower line indicates uterine contractions or perceived fetal movements. This combined recording allows clinicians to correlate changes in the heart rate with periods of movement or uterine tightening, which is fundamental to interpreting the fetus’s overall health status.
Key Parameters of the Fetal Heart Rate Tracing
The interpretation of the NST strip focuses on four specific parameters of the Fetal Heart Rate tracing.
Baseline FHR
The first is the Baseline FHR, which is the average heart rate observed over a ten-minute segment, excluding periods of accelerations or decelerations. A normal baseline rate for a mature fetus generally falls between 110 and 160 beats per minute (bpm). A sustained rate above 160 bpm is classified as tachycardia, while a rate below 110 bpm is known as bradycardia.
Variability
Variability refers to the beat-to-beat fluctuations in the heart rate around the baseline. This fluctuation reflects the healthy interaction between the sympathetic and parasympathetic branches of the fetal nervous system. Moderate variability, defined as a fluctuation range of 6 to 25 bpm, is the most reassuring finding. Minimal variability (less than 5 bpm) or absent variability can indicate potential issues with fetal oxygenation or central nervous system function.
Accelerations
Accelerations are abrupt, temporary increases in the FHR above the established baseline. After 32 weeks of gestation, a healthy acceleration must rise by at least 15 bpm above the baseline and last for 15 seconds or more. Before 32 weeks, the criterion is an increase of at least 10 bpm for 10 seconds. These spontaneous increases are highly predictive of fetal well-being and are typically associated with fetal movement.
Decelerations
The tracing is also analyzed for Decelerations, which are temporary drops in the FHR below the baseline rate. Decelerations are categorized based on their appearance and timing relative to uterine contractions. Early decelerations are usually benign and mirror a contraction, often due to head compression. Late decelerations begin after the contraction has peaked and signal potential uteroplacental insufficiency, making them a significant concern. Variable decelerations are abrupt and irregular, commonly associated with umbilical cord compression.
Determining Reactive Versus Non-Reactive Results
The four key parameters are synthesized to categorize the NST result as either reactive or non-reactive. A Reactive NST is the desired outcome, defined by the presence of at least two qualifying FHR accelerations within a 20-minute monitoring period. This finding demonstrates a healthy response from the fetal nervous system to movement, confirming adequate oxygenation.
If the tracing fails to meet these criteria, it is classified as a Non-Reactive NST. This result does not automatically indicate fetal distress, but it necessitates further evaluation. The most common reason for a non-reactive result is that the fetus is in a natural sleep cycle, which typically lasts 20 to 40 minutes.
Monitoring time is often extended up to 40 minutes to allow the baby to move through a full sleep-wake cycle. If the result remains non-reactive after this extension, it means insufficient reassuring data was collected. A non-reactive result may also occur if the mother has taken medications that suppress fetal central nervous system activity.
Follow-Up After a Non-Reassuring Test
When an NST is non-reactive or concerning decelerations are present, the care team proceeds with additional tests to gather more definitive information. The most frequent follow-up is the Biophysical Profile (BPP), which combines the non-stress test with a comprehensive ultrasound examination. The BPP assigns a score based on five specific criteria:
- The NST result.
- Fetal breathing movements.
- Gross body movements.
- Fetal muscle tone.
- The volume of amniotic fluid.
Another potential follow-up is the Contraction Stress Test (CST), which assesses the fetal heart rate response to artificially induced uterine contractions. The CST provides insight into how the fetus will tolerate the stress of labor, checking for late decelerations that suggest reduced oxygen reserve. These subsequent tests guide healthcare providers on whether to continue expectant management or recommend delivery.