The Non-Stress Test (NST) is a common prenatal screening tool used to monitor the well-being of a developing fetus, typically performed in the third trimester. The test measures the fetal heart rate and its response to movement, looking for temporary increases in heart rate known as accelerations. When a result is described as “non-reactive,” it means the test did not meet the expected criteria within the testing window. While this can cause concern, a non-reactive result is relatively common and often does not signify a serious problem.
Understanding a Non-Reactive Result
A non-reactive NST means the fetus did not exhibit the required accelerations of the heart rate over a 20- to 40-minute period. Specifically, a reactive result requires two or more heart rate increases of at least 15 beats per minute (bpm) above the baseline, lasting 15 seconds or more, within 20 minutes for fetuses over 32 weeks. When these criteria are not met, the result is deemed non-reactive, indicating that the initial snapshot of fetal status is inconclusive and requires further investigation.
The non-reactive finding is interpreted by healthcare providers as a need for more information, not necessarily as evidence of fetal distress. The most frequent reason for a non-reactive result is a temporary state, such as the fetus being in a natural sleep cycle. Fetal sleep states can last up to 40 minutes. Certain maternal medications, like sedatives or some prescription drugs, can also temporarily suppress fetal activity, leading to a non-reactive tracing.
Immediate Follow-Up Testing
Following a non-reactive NST, the next immediate step is typically a more comprehensive evaluation, most commonly the Biophysical Profile (BPP). The BPP combines the NST with a detailed ultrasound examination to assess four additional parameters of fetal health. Each of the five components is scored either 0 or 2 points, resulting in a total possible score of 10.
The five components of a full BPP are:
- The Non-Stress Test (NST)
- Fetal breathing movements
- Gross body movements
- Fetal tone
- The volume of amniotic fluid
A score of 8 or 10 is considered reassuring, while a score of 6 is equivocal and usually necessitates a repeat test within 12 to 24 hours. In some cases, a Contraction Stress Test (CST) may be performed, which involves monitoring the fetal heart rate response to mild uterine contractions. The CST is reserved for situations where the BPP is inconclusive or certain risk factors are present.
Potential Causes for Abnormal Findings
While transient factors often explain a non-reactive NST, persistent abnormal findings on the BPP suggest an underlying issue affecting the fetus’s oxygen supply. The primary concern is often uteroplacental insufficiency, where the placenta cannot adequately transfer oxygen and nutrients to the fetus. This chronic shortage of oxygen, or hypoxia, can suppress the neurological functions responsible for the movements and heart rate accelerations observed during testing.
Certain maternal health conditions can contribute to this placental dysfunction. Uncontrolled maternal hypertension, or high blood pressure, and pre-existing or gestational diabetes mellitus are frequently associated with a higher risk of non-reassuring fetal surveillance results. These conditions can damage the small blood vessels within the placenta, compromising its ability to function efficiently.
Furthermore, an abnormal NST or BPP can be an indicator of Fetal Growth Restriction (FGR), a condition where the fetus is smaller than expected due to inadequate growth. FGR is often a direct result of long-standing placental insufficiency, meaning the non-reassuring test result is a sign that the fetus is adapting its metabolism to cope with a restricted oxygen and nutrient environment. The loss of heart rate reactivity and movement is a protective mechanism to conserve energy in the face of chronic stress.
Management and Intervention Strategies
The course of action following a non-reassuring BPP or CST is determined by the severity of the score, the gestational age of the fetus, and the identified underlying cause. If the BPP score is moderately low, such as a 6, and the fetus is preterm, management may involve continued close monitoring. This monitoring often includes repeating the BPP or NST test twice weekly, along with Doppler flow studies to assess blood flow in the umbilical cord and fetal brain.
If the BPP score is significantly low, such as 4 or less, or if other signs of distress are present, immediate intervention is usually warranted. In such cases, the decision to deliver is made because the risk of remaining in the uterus is greater than the risks associated with prematurity. Delivery may be initiated via induction of labor if the fetal heart rate tracing is stable and delivery is anticipated quickly.
If the fetal tracing is ominously abnormal, displaying recurrent decelerations or profound bradycardia, an expedited delivery via Cesarean section is often necessary to prevent further fetal compromise. The urgency of the delivery is directly related to the degree of oxygen deprivation, with a severe score signaling an immediate threat to the fetus’s health. The goal of all management strategies is to balance the need for more time for fetal maturation with the need to prevent injury from a compromised intrauterine environment.