The Non-Stress Test (NST) is a common, non-invasive procedure used during pregnancy to assess fetal well-being, typically starting after 28 weeks of gestation. This test monitors the baby’s heart rate and how it responds to movement within the womb, providing important information about the baby’s oxygen supply and overall health. The term “non-stress” means the test imposes no physical burden or risk on the mother or the baby.
The Key Components of the Non-Stress Test
Interpreting the Non-Stress Test involves analyzing three specific measurements of the baby’s heart rate pattern. The first measurement is the fetal heart rate baseline, which represents the average rate when the baby is not moving or experiencing a contraction. A normal baseline usually falls between 110 and 160 beats per minute (bpm).
The second component is fetal heart rate variability, which refers to the small, irregular fluctuations in the heart rate around the baseline. Moderate variability is the desired pattern, characterized by changes of 6 to 25 bpm, and it suggests a healthy, well-functioning nervous system in the baby. Minimal variability, where the range is 5 bpm or less, can sometimes indicate the baby is sleeping or may suggest a need for further evaluation.
The third and most telling component is the presence of accelerations, which are temporary increases in the fetal heart rate. The absence or presence of decelerations, which are temporary decreases in the heart rate, is also noted, as certain types can indicate a need for concern.
Defining a Reassuring (“Good”) NST Result
A test is classified as “reassuring” or “reactive” when the components indicate that the fetus is adequately oxygenated and neurologically healthy. This result requires a normal baseline heart rate and the presence of moderate variability.
The primary criterion for a reactive result is the occurrence of at least two accelerations within a 20-minute monitoring period. For pregnancies past 32 weeks, an acceleration is defined by the “15×15 rule,” meaning the heart rate must increase by at least 15 bpm above the baseline and last for 15 seconds. If the pregnancy is before 32 weeks, the standard is slightly adjusted to a 10 bpm increase lasting 10 seconds.
A reactive result is a strong indicator of adequate fetal oxygenation because a lack of oxygen would suppress the baby’s ability to produce these heart rate increases. This outcome is reassuring and typically means no immediate intervention is required.
Interpreting Non-Reassuring Results and Next Steps
A test is termed “non-reactive” if it fails to meet the required criteria for accelerations within the initial monitoring period. It is important to understand that a non-reactive result does not automatically signify a problem with the baby’s health. The most common reason for a non-reactive test is that the baby was in a natural sleep cycle or was otherwise inactive during the assessment.
When a non-reactive result occurs, the immediate next step is often to extend the monitoring period, sometimes up to 40 minutes, to allow time for the baby to wake up. Healthcare providers may also use a small buzzing device on the mother’s abdomen, known as acoustic stimulation, to gently encourage movement and wakefulness.
If the test remains non-reactive after these steps, it suggests that further investigation is necessary to rule out any concerns about the baby’s oxygen status. The provider will then typically proceed to a secondary assessment, such as a Biophysical Profile (BPP). This follow-up testing provides a more comprehensive picture by combining the NST results with an ultrasound examination of other factors like breathing, movement, tone, and amniotic fluid volume.