The Non-Stress Test (NST) is a common, non-invasive procedure used in prenatal care to evaluate the well-being of an unborn baby. The test observes the baby’s heart rate patterns to provide information about fetal oxygen supply. It is a painless measurement that helps healthcare providers ensure the fetus is receiving adequate blood flow and oxygenation from the placenta. Understanding how to interpret the data can help expectant parents feel informed about their baby’s health status.
The Purpose and Procedure of the Non-Stress Test
The primary reason an NST is ordered is to monitor pregnancies carrying a higher risk, or those that have progressed beyond the due date. Conditions such as maternal diabetes, high blood pressure, or decreased fetal movement often prompt this screening. The test is typically performed after 28 weeks of gestation, when the fetal nervous system is mature enough to show expected heart rate responses.
The name “non-stress” indicates the test places no physical strain on the fetus or the mother. During the procedure, the mother reclines while two external monitors are secured to her abdomen with belts. A tocodynamometer detects uterine contractions, and an ultrasound transducer records the baby’s heart rate and movement.
The standard testing period lasts for a minimum of 20 minutes, but may be extended to 40 minutes if the baby is in a sleep cycle. The mother may press a button each time she senses movement, correlating fetal activity with the recorded heart rate tracing.
Interpreting Fetal Heart Rate Patterns
Interpreting an NST involves analyzing three specific features of the fetal heart rate tracing. The first is the Baseline Heart Rate, which is the average rate when the baby is at rest. For a healthy fetus, this baseline usually falls within 110 to 160 beats per minute (BPM).
The second feature is Variability, referring to the slight, irregular fluctuations in the heart rate around the baseline. These subtle changes appear as a jagged line and signal a healthy, functioning nervous system. Moderate variability, defined as an amplitude range between 6 and 25 BPM, indicates fetal well-being and adequate oxygenation.
The third component is the presence of Accelerations, which are temporary increases in the heart rate above the baseline. These increases are the expected response to movement and confirm the baby is receiving sufficient oxygen.
The criteria for a significant acceleration depend on gestational age. At 32 weeks or beyond, a qualifying acceleration must rise at least 15 BPM above the baseline and last for a minimum of 15 seconds. For fetuses under 32 weeks, the threshold is 10 BPM for a duration of 10 seconds.
Understanding Reactive and Non-Reactive Results
The final interpretation relies on whether the required number of accelerations was observed. A Reactive result is considered reassuring for fetal health. This means the baby met the established criteria by displaying at least two qualifying heart rate accelerations during the 20-minute test.
A reactive test confirms the fetus has a healthy nervous system and is likely well-oxygenated, as the heart rate increased appropriately in response to movement. The presence of moderate variability alongside these accelerations supports the conclusion of fetal health.
Conversely, a Non-Reactive result means the baby did not show the two required accelerations within the initial 20 minutes. This finding does not automatically indicate a serious problem; the most common reason is that the baby was in a normal sleep cycle.
Maternal medications can also cause the baby to be sedated or less active, contributing to a non-reactive outcome. The absence of the required response necessitates further assessment to confirm fetal well-being.
Next Steps Following a Non-Reactive Result
When an NST yields a non-reactive result, the initial step is to stimulate the baby to encourage movement and heart rate increase. The test may be extended for an additional 20 minutes, totaling 40 minutes, to account for a fetal sleep cycle. Providers may also use an acoustic stimulator placed on the abdomen to wake the baby.
If extended monitoring and stimulation do not produce a reactive pattern, a secondary test is ordered for a more comprehensive assessment. The most common follow-up is a Biophysical Profile (BPP), which combines the NST with an ultrasound examination. The BPP evaluates factors including fetal breathing movements, body movements, muscle tone, and amniotic fluid volume.
In some cases, a Contraction Stress Test (CST) may be performed, assessing how the baby’s heart rate responds to mild uterine contractions. These additional tests gather data to help the care team determine the best course of action, which may involve closer monitoring or, rarely, early delivery.