The Non-Stress Test (NST) is a common, non-invasive method used to monitor fetal well-being during pregnancy. This test assesses how the baby’s heart rate responds to movement, providing an indicator of their current health and oxygen supply. Interpreting the NST involves recognizing specific patterns in the fetal heart rate tracing to determine if the baby is thriving. Results are categorized as either “reactive” or “non-reactive,” classifications that guide further clinical decisions in prenatal care.
Purpose and Procedure of the Non-Stress Test
The NST is performed primarily to assess fetal health, especially in higher-risk pregnancies involving conditions like diabetes or high blood pressure. It is also used when a pregnancy extends past the due date or if the parent reports decreased fetal movement. The underlying principle is that a healthy baby’s heart rate temporarily increases when they move, indicating adequate oxygenation and a functioning nervous system.
During the procedure, the parent reclines while two elastic belts with sensors are placed around the abdomen. One sensor, an ultrasound transducer, tracks the fetal heart rate (FHR), and the other monitors uterine contractions. The test typically lasts a minimum of 20 minutes but may be extended to 40 minutes if the baby is inactive, possibly due to a sleep cycle. The parent may be asked to press a button to correlate fetal movement with recorded heart rate changes.
Interpreting Fetal Heart Rate Accelerations and Variability
Interpreting the NST begins with establishing the baseline Fetal Heart Rate (FHR)—the average rate when the baby is inactive. A normal baseline FHR for a term fetus ranges between 110 and 160 beats per minute (bpm). A rate outside this range may warrant closer observation.
Variability, the small, normal fluctuations in heart rate from beat to beat, is another important measure. Moderate variability is a healthy sign, reflecting the balanced activity of the nervous systems controlling the heart. Good variability is often considered the most telling sign of an adequately oxygenated fetus.
Accelerations are temporary, abrupt increases in the FHR above the baseline. These are positive indicators that the fetal nervous system is receiving sufficient oxygen to respond to activity. For pregnancies at 32 weeks gestation or greater, a significant acceleration rises by at least 15 bpm and lasts for 15 seconds or more.
Defining Reactive and Non-Reactive Test Outcomes
The final NST classification depends on whether specific acceleration criteria are met during the observation period. A Reactive NST is a reassuring result, confirming the baby is well-oxygenated and their neurological system is functioning. To be considered reactive, the tracing must show at least two accelerations meeting the “15 by 15” criteria within a 20-minute period.
A Non-Reactive NST means the test did not meet the minimum criteria for reactivity within the standard testing window. This outcome does not automatically indicate a problem, as the baby may have been in a deep sleep state or the result may be caused by certain parental medications. However, a non-reactive result may also suggest an issue with oxygen supply.
If the baby is inactive, the monitoring period is often extended up to 40 minutes to allow the baby a chance to awaken. If the acceleration criteria are still not met after this extended period, the result remains non-reactive and requires further evaluation.
What Happens After a Non-Reactive Result?
If an NST remains non-reactive after the extended monitoring period, the healthcare team pursues additional diagnostic steps to assess fetal health. The common next step is often a Biophysical Profile (BPP), which provides a more comprehensive assessment of the baby’s well-being. The BPP combines the non-stress test data with an ultrasound examination of four other biophysical variables.
The ultrasound portion of the BPP scores four variables: fetal breathing movements, overall body movement, muscle tone, and the volume of amniotic fluid. Each of the five components (including the NST) is assigned a score. A total score of 8 to 10 out of 10 is considered normal and reassuring.
Another potential follow-up is the Contraction Stress Test (CST), which observes the FHR response to induced or spontaneous uterine contractions. The CST is typically avoided if there are reasons the parent should not go into labor. These subsequent tests help determine if intervention is needed or if the non-reactive result was temporary.