A Non-Stress Test (NST) is a common prenatal assessment used to evaluate fetal well-being. It provides insight into the baby’s health by monitoring heart rate patterns in response to movement. This article aims to clarify the components of an NST strip and explain how its results are interpreted.
What is a Non-Stress Test (NST)?
A Non-Stress Test is a safe and non-invasive procedure performed during pregnancy, typically after 28 weeks of gestation, when the fetal heart rate begins reacting to movements. It helps determine if the baby is receiving sufficient oxygen and responding appropriately. It is often recommended in situations such as reduced fetal movement, overdue pregnancies, or in cases of high-risk conditions like high blood pressure or diabetes. The name “non-stress” indicates that the test places no additional stress on either the pregnant individual or the fetus.
During an NST, two belt-like devices are placed around the abdomen while the individual lies in a reclining position. One device measures the fetal heart rate, while the other records uterine contractions. The test typically lasts between 20 and 30 minutes. If the baby is not active, a healthcare provider may use a gentle sound device or suggest a snack to encourage movement.
Understanding the NST Strip Components
The NST strip provides a visual representation of the fetal heart rate and uterine activity over time, displaying several components. One primary element is the Baseline Fetal Heart Rate (FHR), which represents the average heart rate of the fetus when it is not experiencing accelerations or decelerations. A normal baseline FHR typically ranges between 110 and 160 beats per minute (bpm). This baseline rate can fluctuate slightly throughout the day due to fetal activity.
Another important aspect is Variability, which refers to the irregular fluctuations in the baseline FHR. Variability reflects the interplay between the fetal nervous system and heart, indicating a well-oxygenated fetus. It is quantified by the amplitude of these fluctuations. Variability is categorized as absent (undetectable), minimal (detectable but 5 bpm or less), moderate (6-25 bpm), or marked (greater than 25 bpm). Moderate variability is considered a sign of fetal well-being.
Accelerations are temporary increases in the fetal heart rate above the baseline. These are considered a positive indicator of fetal health. For fetuses at 32 weeks gestation or beyond, an acceleration is defined as an increase of at least 15 bpm above the baseline, lasting for 15 seconds or more. For those under 32 weeks, the criteria are an increase of at least 10 bpm for 10 seconds or more.
Conversely, Decelerations are temporary decreases in the fetal heart rate from the baseline. There are different types, each with varying implications.
Early decelerations are gradual, symmetrical drops in FHR that mirror uterine contractions, occurring at the same time as the contraction’s peak. They are considered normal and often result from fetal head compression.
Late decelerations are also gradual but begin after the contraction starts and return to baseline after the contraction ends. These can sometimes indicate decreased blood flow to the placenta.
Variable decelerations are abrupt, irregular drops in FHR, varying in their timing relative to contractions, and can be caused by umbilical cord compression. While some decelerations can be normal, recurrent decelerations may warrant further evaluation.
Interpreting NST Results
The information gathered from the NST strip is then used to classify the test as either reactive (reassuring) or non-reactive (non-reassuring), which helps guide further management. A Reactive NST indicates that the fetus is healthy and well-oxygenated. For a test to be considered reactive, there must be at least two accelerations within a 20-minute period. The presence of moderate variability and the absence of concerning decelerations further support a reactive classification.
A Non-reactive NST means that the test did not meet the criteria for a reactive result, showing insufficient accelerations or a lack of expected heart rate increases with movement. This outcome does not automatically signal a problem with the baby. A non-reactive result can occur if the baby is asleep during the test, or if certain medications have been taken. It might suggest that the baby is not getting enough oxygen, but this is not always the case. A non-reactive test indicates that more information is needed and prompts further investigation.
Next Steps After an NST
When an NST result is non-reactive, it serves as a signal for healthcare providers to conduct further assessments to ensure fetal well-being. Common follow-up procedures include extending the NST for a longer duration, up to 40 minutes or more, to observe the baby outside a sleep cycle.
Another frequent next step is a Biophysical Profile (BPP). A BPP combines the NST with an ultrasound to evaluate several aspects of fetal health. The ultrasound assesses:
Fetal breathing movements
Body movements
Muscle tone
The amount of amniotic fluid
Each of these five components receives a score, providing a comprehensive picture of the baby’s condition.
A Contraction Stress Test (CST) may be recommended. This test observes the baby’s heart rate response during uterine contractions, which can be naturally occurring or induced with medication. The CST helps determine how well the fetus will tolerate the stress of labor. These additional tests are part of a thorough evaluation process for the health of both the pregnant individual and the baby.