Nonstress Test: Purpose, Test Details & Results

Purpose of Nonstress Test

The journey of pregnancy is filled with anticipation, hope, and—sometimes—concern about the baby’s well-being. For expectant parents and healthcare providers, ensuring the health of the fetus, especially in high-risk pregnancies, is paramount. The nonstress test (NST) is a widely used, noninvasive tool that helps monitor fetal health by assessing the baby's heart rate response to its own movements. This section explores why the NST is performed, whom it helps most, and what clinical questions it answers.

Table 1: Purpose and Clinical Use of the Nonstress Test

Understanding the Need for Fetal Monitoring

Pregnancy can be complicated by conditions such as hypertension, diabetes, intrauterine growth restriction, or decreased fetal movement. These situations increase the risk of fetal distress or poor outcomes. The NST was developed as a solution for noninvasive, rapid, and repeatable assessment of fetal health, focusing on heart rate patterns in relation to fetal movement 2 5 6.

Who Needs the NST?

While any pregnant individual may undergo an NST, it is most often recommended for those considered “high-risk,” including:

• Post-date pregnancies (beyond the due date)
• Those with decreased fetal movement noted
• Mothers with diabetes, hypertension, or suspected fetal growth restriction 1 2

Clinical Goals of the NST

The primary aim is to determine if the fetus is getting enough oxygen and can respond appropriately to its environment. A reactive NST, showing healthy heart rate accelerations with movement, is a reassuring sign and typically predicts good outcomes 2 5 6 7. In contrast, a nonreactive or abnormal result may prompt further investigations or even early delivery to prevent complications 1 2 7.

Impact on Pregnancy Outcomes

Research demonstrates that, while the NST is valuable in detecting fetuses at risk, its use must be part of a broader strategy. For example, studies comparing NST with other tests, such as the umbilical artery Doppler, indicate that while both can be effective, test selection can influence outcomes like cesarean delivery rates for fetal distress 1. Nonetheless, the NST remains a cornerstone in fetal surveillance, particularly in high-risk groups 2 5 6.

Nonstress Test Details

The NST is as much about its simplicity as its clinical power. Here, we break down how the test is performed, the science behind it, and innovations that have improved its reliability and efficiency.

Table 2: How the Nonstress Test is Performed and Interpreted

How is the NST Performed?

A typical NST involves the mother reclining in a comfortable position while two monitors are placed on her abdomen:

• One sensor tracks the fetal heart rate.
• The other detects uterine contractions or fetal movements.

The goal is to observe whether the fetus’s heart rate accelerates appropriately when it moves—a healthy sign of oxygenation and nervous system function 2 5 6.

What Happens During the Test?

• The test usually lasts 20 minutes, but may be extended if the baby is inactive (“asleep”) during the session 2 6 7.
• If the baby doesn’t move much, gentle stimulation (like sound or vibration) may be used to “wake” the fetus and provoke movement 3 4 6.
• The mother may be asked to press a button when she feels movement, correlating her perception with the heart rate tracing.

Innovations and Adjuncts

Acoustic Stimulation

To reduce the number of nonreactive tests and shorten the time required for results, clinicians sometimes use acoustic stimulation. A brief sound is applied to the abdomen to wake the fetus, which may prompt a quick, reliable heart rate acceleration 3 4 6. Studies have shown this can decrease the proportion of nonreactive tests and make the process more efficient, without compromising safety 3 4.

Rapid NST Protocols

Research has led to shorter NSTs by using stimulation and focusing on the most meaningful indicators. This can be especially helpful when fetal activity is initially low, allowing clinicians to quickly differentiate between healthy and concerning patterns 4 6.

Limitations and Reliability

While the NST is considered simple and noninvasive, interpretation can vary between clinicians. Interobserver (between different doctors) and intraobserver (same doctor at different times) agreement on NST results is not perfect, with some variability in readings—this can affect sensitivity and the need for additional testing 8. Proper training and clear criteria can help improve reliability.

Nonstress Test Results & Follow-Up

Once the NST is complete, the results guide next steps. Understanding what the results mean—and what to do if they’re not clearly reassuring—is essential for both healthcare providers and families.

Table 3: Interpreting NST Results and Recommended Actions

What Do NST Results Mean?

Reactive (Normal)

A "reactive" NST means there are at least two accelerations of fetal heart rate within a 20-minute window, each lasting at least 15 seconds and rising at least 15 beats per minute above the baseline (for fetuses over 32 weeks). This result is highly reassuring, correlates with good fetal oxygenation, and is predictive of positive pregnancy outcomes 2 5 6 7.

Nonreactive

A "nonreactive" NST lacks the required accelerations. This may be due to fetal sleep, immaturity, or—more concerningly—fetal compromise. However, a single nonreactive test does not always mean the fetus is in danger. Often, the test is repeated, possibly with acoustic stimulation or after feeding the mother, to rouse the baby. Persistent nonreactive results prompt further evaluation, such as:

• Contraction stress test (CST)
• Biophysical profile (ultrasound-based)
• Doppler studies of umbilical blood flow 1 2 5 6 7

Nonreactive NSTs are associated with higher rates of cesarean section for fetal distress and increased perinatal mortality, underlining the need for prompt follow-up 2 7.

Equivocal or Unsatisfactory

Sometimes, test results are ambiguous or technically inadequate—perhaps due to maternal movement, poor sensor contact, or inability to interpret the tracing. In these cases, the NST is typically repeated 2 8.

Next Steps After Abnormal or Equivocal Results

• Stimulation: Gentle movement of the maternal abdomen or acoustic stimulation may be attempted first, often converting a nonreactive to a reactive test 3 4 6 7.
• Further Testing: If the NST remains nonreactive, CST or an ultrasound-based biophysical profile may be performed to further assess fetal well-being 1 2 5 6 7.
• Intervention: If additional testing confirms fetal compromise, early delivery may be considered to prevent adverse outcomes 1 2.

Variability in Interpretation

Studies have highlighted that different clinicians may interpret the same NST differently, which can affect decision-making. Consistent training, clear protocols, and adjunctive tests help reduce the risk of misinterpretation 8.

Conclusion

The nonstress test is a vital, noninvasive method for assessing fetal well-being, especially in high-risk pregnancies. Its simplicity, safety, and ability to provide real-time reassurance make it a cornerstone of prenatal care. However, understanding its limitations and the importance of follow-up ensures that both clinicians and families can make informed decisions for the healthiest possible outcomes.

Key takeaways:

• NST Purpose: Primarily used in high-risk pregnancies to assess fetal health and guide interventions if needed 1 2 5 6 7.
• Test Details: Involves monitoring fetal heart rate in response to movement, occasionally enhanced with acoustic or tactile stimulation for efficiency 3 4 6.
• Results & Follow-Up: Reactive results are reassuring, while nonreactive or equivocal results require further testing to ensure fetal safety 1 2 5 6 7 8.
• Limitations: Variability in interpretation exists; standardized criteria and adjunctive techniques help improve reliability 8.
• Clinical Impact: When properly utilized, the NST helps reduce adverse outcomes and guides timely interventions in pregnancies at risk 1 2 7.

By combining thoughtful application, innovative adjuncts, and careful interpretation, the NST remains an essential tool for safeguarding babies before birth.