How to Read an NST: Nonstress Test, Fetal Heart Rate Monitoring, Contractions, and Prenatal Fetal Assessment

The nonstress test (NST) is an important part of prenatal care that helps assess fetal well-being by observing the relationship between fetal movement and heart rate patterns. During this noninvasive prenatal test, a fetal monitor records the fetal heart rate and may also record uterine activity, allowing healthcare professionals to evaluate how the fetus responds to movement. In obstetrics, these findings provide valuable information about fetal status and help guide decisions about whether routine monitoring is sufficient or further fetal assessment is needed.

Understanding how to read an NST requires familiarity with several key features of fetal heart rate monitoring, including baseline heart rate, variability, accelerations, and decelerations. These findings help determine whether an NST is reactive or nonreactive, but they must be interpreted in the context of gestational age, fetal sleep cycles, maternal health, and the reason for testing. For example, a reactive NST generally indicates a reassuring fetal heart rate response to movement, while a nonreactive result may occur because the fetus is sleeping or because additional evaluation is necessary. A nonreactive NST alone does not confirm fetal distress, and further assessment may be needed before conclusions are drawn.

The NST is also part of a broader approach to antepartum fetal surveillance, particularly in pregnancies where conditions such as hypertension, gestational diabetes, or suspected fetal growth restriction may increase the need for monitoring. Its findings may be considered alongside other assessments, including the biophysical profile, which combines fetal observations with ultrasound measurements, or the contraction stress test, which evaluates fetal heart rate responses to uterine contractions. Understanding the purpose and limitations of each test helps clinicians interpret results within the wider clinical picture rather than relying on a single finding.

A systematic approach to reading an NST involves recognizing normal fetal heart rate patterns, understanding the significance of fetal movement and uterine activity, and identifying results that require follow-up. Accurate interpretation supports clear documentation, effective communication with the healthcare team, and timely escalation when findings are concerning. By understanding how the NST works and what its results can and cannot establish, healthcare professionals can contribute to evidence-based prenatal care and the ongoing assessment of fetal health.

Nonstress Test (NST): Purpose and Prenatal Use

The nonstress test (NST) is an important component of antepartum fetal surveillance, particularly when a pregnancy requires closer monitoring because of maternal health conditions or concerns about fetal well-being. It provides information about how the fetal heart rate responds to fetal movement, helping healthcare professionals identify reassuring patterns and determine whether further assessment may be necessary. Understanding the purpose and clinical use of the NST is an essential part of obstetrics because it helps place fetal monitoring findings within the broader context of prenatal care.

Definition and Clinical Purpose of the Nonstress Test

A nonstress test is a noninvasive prenatal test that monitors the fetal heart rate over a period of time, usually while the fetus is at rest or moving naturally. The term nonstress means that the test does not deliberately induce uterine contractions or place physical stress on the fetus. Instead, it observes the natural relationship between fetal activity and changes in heart rate. A fetal monitor records the heart rate, allowing the healthcare provider to assess whether expected accelerations occur during fetal movement. 

The underlying principle is that a healthy fetus with adequate oxygenation and a functioning autonomic nervous system will generally demonstrate temporary increases in heart rate when moving. These increases are known as accelerations. They reflect the interaction between fetal movement, neurological regulation, and cardiovascular response. When the fetal heart rate accelerates appropriately, the finding is generally reassuring about the fetus’s condition at the time of testing.

The NST does not directly measure the amount of oxygen reaching the fetus, nor does it provide a complete picture of every aspect of fetal health. Rather, it evaluates an indirect physiological sign that can help clinicians assess fetal well-being. The results must therefore be interpreted alongside gestational age, maternal medical history, fetal movement, and any other relevant clinical findings.

A typical NST involves placing an electronic sensor over the pregnant person’s abdomen to record the fetal heart rate. A second sensor may detect uterine activity, while a button or other recording method may allow fetal movements to be marked on the tracing. The resulting record enables the healthcare professional to evaluate changes in heart rate over time rather than relying on a single measurement.

Several important features help explain the clinical purpose of the NST:

• Assessment of fetal heart rate response: The test examines whether the fetal heart rate increases appropriately with movement and whether the observed pattern is reassuring.
• Identification of reactive findings: A reactive NST demonstrates the required accelerations for the applicable gestational age and testing criteria. In a typical pregnancy at 32 weeks or later, this commonly means at least two accelerations of 15 beats per minute or more above baseline, each lasting at least 15 seconds, within a 20-minute period.
• Recognition of nonreactive findings: If the required accelerations are not observed within the initial monitoring period, the result may be classified as nonreactive. This does not automatically indicate a problem because fetal sleep, prematurity, medications, and other factors may influence the tracing.
• Guidance for further assessment: Depending on the findings and clinical circumstances, the healthcare provider may recommend continued monitoring, repeat testing, a biophysical profile, or another appropriate assessment.

For example, consider a pregnant patient at 34 weeks of gestation who attends a scheduled prenatal appointment because of a medical condition that increases the risk of pregnancy complications. During the NST, the fetal heart rate shows a stable baseline and two appropriate accelerations associated with fetal movement within the observation period. This would generally meet the criteria for a reactive NST at that gestational age. The finding is reassuring at the time of testing, although it does not guarantee that complications cannot develop later.

In contrast, if the fetal heart rate does not show the required accelerations, the result may be nonreactive. The fetus may be asleep, or the response may be affected by gestational age or medication. The healthcare provider may extend the monitoring period or arrange additional testing to determine whether the findings need further investigation. A nonreactive result is therefore a reason to evaluate the clinical picture, not a diagnosis of fetal distress on its own.

Indications for NST During Pregnancy

An NST is not routinely required in every uncomplicated pregnancy. It is most commonly used when a maternal condition, fetal finding, or obstetric circumstance raises concern about the possibility of complications. The decision to perform this prenatal test depends on the specific risk factor, its severity, the gestational age, and whether the results could influence clinical management.

The American College of Obstetricians and Gynecologists (ACOG) identifies several circumstances in which antepartum fetal surveillance may be appropriate. The timing and frequency of testing should be individualized because the evidence supporting surveillance varies across conditions. 

1. High-risk pregnancies and maternal medical conditions

Pregnancies affected by certain medical conditions may require closer assessment because these conditions can affect placental function, fetal growth, or the supply of oxygen and nutrients to the fetus. An NST may be included in the surveillance plan when the expected benefits of monitoring justify its use.

Relevant conditions include:

• Hypertension and preeclampsia: High blood pressure during pregnancy can be associated with placental dysfunction and impaired fetal growth. Depending on severity and gestational age, an NST may help assess fetal status alongside maternal blood pressure, laboratory findings, fetal growth, and other investigations.
• Pregestational diabetes and gestational diabetes: Diabetes can increase the risk of adverse pregnancy outcomes, particularly when glucose control is poor or other complications are present. Fetal surveillance may be recommended according to glycemic control, treatment requirements, and associated maternal or fetal conditions.
• Chronic kidney disease and other significant maternal illnesses: Some chronic conditions can affect placental function or increase the risk of complications, making individualized fetal surveillance appropriate.
• Multiple maternal or obstetric risk factors: When several risk factors occur together, the overall risk may be greater than that associated with an individual condition. The healthcare team considers the combined clinical picture when deciding whether and when to begin monitoring.

For example, a pregnant patient with chronic hypertension and suspected fetal growth restriction may require closer monitoring than someone with well-controlled hypertension and normal fetal growth. The NST may form part of that assessment, alongside ultrasound and, when indicated, umbilical artery Doppler studies. A reassuring NST does not replace these other investigations.

2. Decreased fetal movement

A noticeable reduction in fetal movement may prompt assessment because changes in movement can sometimes accompany fetal compromise. When a pregnant person reports that the fetus is moving less than usual, the healthcare provider may recommend an NST to examine the fetal heart rate response and determine whether additional evaluation is necessary.

During testing, the clinician assesses whether fetal movement is accompanied by appropriate heart rate accelerations. If the fetus does not move much during the initial observation period, sleep may be one explanation. However, reduced movement reported by the pregnant person should not automatically be attributed to fetal sleep, and a nonreactive result may require further investigation.

For example, a patient at 35 weeks of gestation who notices that the baby is moving substantially less than usual should contact their maternity care team promptly. Depending on the assessment and local clinical protocol, fetal heart rate monitoring and other investigations may be arranged. They should not wait until a scheduled NST appointment if movement has decreased significantly. 

3. Suspected fetal growth restriction

Fetal growth restriction occurs when the fetus does not reach its expected growth potential. It may be associated with placental insufficiency, maternal hypertension, or other conditions that affect fetal development. Because some affected fetuses have an increased risk of stillbirth, antenatal surveillance may be recommended.

The NST can provide information about fetal heart rate reactivity, but it does not measure fetal size or diagnose growth restriction. Ultrasound measurements are used to evaluate growth, while Doppler studies may assess blood flow through the umbilical artery and other vessels when clinically indicated. The findings are considered together to guide surveillance and decisions about the timing of delivery. 

4. Pregnancy continuing beyond the expected due date

When pregnancy continues beyond the expected due date, fetal surveillance may be recommended because certain risks, including stillbirth, increase as gestation advances. The decision to begin NST monitoring depends on gestational age, the clinical circumstances, and the plan for ongoing pregnancy management.

For an otherwise uncomplicated pregnancy that reaches 41 weeks, ACOG guidance supports considering or recommending antenatal fetal surveillance, with the timing and frequency determined by the clinical situation. An NST may be used alone or alongside an assessment of amniotic fluid volume. The results help inform care but do not, by themselves, determine whether delivery is necessary.

5. Multiple pregnancy and other obstetric complications

Pregnancies involving twins or higher-order multiples may require additional surveillance, particularly when there are concerns about unequal fetal growth, placental complications, or other factors that increase risk. The appropriate approach depends on the type of multiple pregnancy, gestational age, ultrasound findings, and maternal health.

Other circumstances that may prompt consideration of fetal surveillance include certain placental or amniotic fluid abnormalities and a history or clinical finding associated with increased risk of adverse pregnancy outcomes. However, the presence of any single risk factor does not automatically mean that an NST is required. The healthcare provider determines whether monitoring is appropriate and whether other methods of fetal assessment are needed. 

6. Determining when to begin and how often to repeat NST testing

The gestational age at which NST monitoring begins depends on the condition being monitored and the likelihood that an abnormal result would lead to a meaningful clinical intervention. For many pregnancies requiring surveillance, testing may begin around 32 weeks, but some conditions warrant earlier assessment, while others may not require testing until later in pregnancy.

The frequency of testing also varies. Depending on the clinical circumstances, an NST may be performed weekly, twice weekly, or more frequently when additional concerns arise. These schedules are not universal; they should reflect the maternal and fetal condition, previous results, other test findings, and applicable clinical guidance. 

For example, a patient with a stable medical condition and reassuring fetal assessments may follow a scheduled surveillance plan. If fetal movement decreases, maternal hypertension worsens, or a previous test produces concerning findings, the healthcare team may reassess the timing and type of further monitoring.

The central purpose of ordering an NST is to obtain clinically useful information about fetal status when a pregnancy warrants additional surveillance. The test supports, rather than replaces, a comprehensive assessment that considers maternal symptoms, fetal movement, gestational age, and other relevant findings. Its results should always be interpreted in context, with further assessment arranged when indicated.

How to Read an NST: Fetal Heart Rate Patterns and NST Results

Learning how to read an NST involves examining the fetal heart rate tracing systematically rather than relying on a single number or isolated change. The interpretation focuses on the baseline fetal heart rate, variability, accelerations, and decelerations, together with the overall pattern observed during the monitoring period. These features help determine whether the NST results are reassuring or whether further fetal assessment is needed.

The nonstress test evaluates the fetal heart rate in response to fetal movement over time. A reactive result is generally reassuring at the time of testing, while a nonreactive result means that the required accelerations have not been observed within the assessment period. Neither result should be interpreted without considering gestational age, fetal activity, maternal conditions, and the reason the test was ordered.

Baseline Fetal Heart Rate and Variability

The baseline fetal heart rate is the average rate at which the fetal heart beats over a defined period, excluding temporary increases and decreases associated with accelerations and decelerations. It provides the starting point for interpreting other heart rate patterns and helps establish whether the fetal heart rate is within the expected range.

In standard fetal heart rate terminology, the baseline is estimated over a 10-minute window and rounded to increments of five beats per minute. At least two minutes of identifiable baseline must be present within that window for a baseline to be assigned. If there is insufficient information, the clinician may need to review a longer portion of the tracing. 

Normal baseline fetal heart rate

For most pregnancies, a baseline fetal heart rate of 110–160 beats per minute (bpm) is considered normal. The baseline must be distinguished from temporary increases during movement and temporary decreases that may occur during the monitoring period.

For example, if the fetal monitor shows a baseline of approximately 140 bpm, with brief increases to 160 bpm when the fetus moves, the baseline remains 140 bpm. The temporary increases are assessed separately as potential accelerations.

Two baseline findings that require attention are:

• Fetal bradycardia: A baseline below 110 bpm that persists for at least 10 minutes.
• Fetal tachycardia: A baseline above 160 bpm that persists for at least 10 minutes.

These definitions refer to sustained baseline changes, not brief fluctuations. A single low or high reading does not establish either condition. The healthcare provider must verify the tracing and assess the wider clinical context. 

A persistently elevated fetal heart rate may be associated with maternal fever, infection, certain medications, or other fetal and maternal conditions. A persistently low baseline may have several explanations, including fetal cardiac rhythm abnormalities or other clinical circumstances. Neither finding should be interpreted in isolation, and the appropriate response depends on the entire assessment.

Fetal heart rate variability

Variability describes the small, irregular fluctuations in the fetal heart rate around its baseline. These changes reflect the interaction of the fetal autonomic nervous system with cardiovascular regulation. Variability is an important feature because it provides information about fetal physiological responsiveness.

Standard terminology classifies baseline variability into four categories:

Moderate variability is generally a reassuring feature of fetal heart rate monitoring. It indicates that the heart rate is changing in a way consistent with ongoing autonomic regulation. However, variability must be interpreted alongside accelerations, decelerations, and other clinical findings.

Minimal variability may occur when the fetus is in a sleep cycle, particularly during periods of reduced activity. It can also be associated with prematurity, medications, or fetal compromise. Absent variability, especially when accompanied by recurrent decelerations or persistent bradycardia, is more concerning and requires prompt clinical evaluation. Marked variability also needs to be interpreted in context rather than automatically classified as reassuring or abnormal.

For example, a tracing with a baseline of 135 bpm, moderate variability, and appropriate accelerations during movement contains several reassuring features. By contrast, a tracing with absent variability and recurrent decelerations would warrant urgent assessment by the clinical team.

How to interpret baseline and variability together

When learning how to read an NST, assess the baseline and variability before interpreting individual accelerations or decelerations. A practical sequence is:

1. Identify the baseline fetal heart rate and determine whether it is within the expected range.
2. Assess the variability around the baseline.
3. Identify accelerations associated with fetal activity.
4. Look for decelerations or other concerning changes.
5. Interpret the combined pattern in relation to gestational age and the reason for monitoring.

This approach helps prevent a common error: assuming that a normal baseline alone proves that fetal well-being is satisfactory. The baseline is only one part of the tracing, and the overall NST interpretation depends on the pattern as a whole

Accelerations and Reactive NST Results

Accelerations are temporary increases in the fetal heart rate above the established baseline. They are particularly important when learning how to read an NST because they demonstrate the fetal heart’s response to movement and provide evidence of neurological and cardiovascular responsiveness.

When a fetus moves, its heart rate commonly increases briefly. This response is influenced by the autonomic nervous system and is generally reassuring when it occurs according to the criteria appropriate for gestational age. The NST evaluates these changes over time to determine whether the required accelerations are present. 

Criteria for identifying fetal heart rate accelerations

The standard criteria for an acceleration depend on gestational age:

• At 32 weeks of gestation or later: The fetal heart rate increases by at least 15 bpm above baseline and remains elevated for at least 15 seconds but less than two minutes.
• Before 32 weeks of gestation: An acceleration is generally defined as an increase of at least 10 bpm above baseline lasting at least 10 seconds.

These are commonly known as the 15-by-15 and 10-by-10 criteria, respectively. The lower threshold in earlier gestation accounts for developmental differences in fetal heart rate responses. 

For example, consider a fetus at 34 weeks of gestation with a baseline heart rate of 140 bpm. During movement, the heart rate rises to 158 bpm and remains elevated for 20 seconds before returning to baseline. This is an acceleration because the heart rate has increased by 18 bpm for longer than 15 seconds.

If the same fetus demonstrates two qualifying accelerations within the assessment period, the NST may meet the criteria for a reactive result, provided the tracing is technically adequate and the findings are interpreted in the appropriate clinical context.

What defines a reactive NST?

A reactive NST is one in which the required number of qualifying fetal heart rate accelerations occurs within the specified monitoring period. For a pregnancy at 32 weeks or later, the usual criterion is at least two accelerations of 15 bpm or more above baseline, each lasting at least 15 seconds, within a 20-minute period. 

A reactive NST is generally considered reassuring because the fetal heart rate has demonstrated the expected response during the test. It suggests that, at the time of monitoring, there is no evidence from this assessment of a problem requiring immediate further investigation solely because of absent reactivity.

A reactive result does not guarantee that the fetus will remain well indefinitely. It also does not rule out every possible fetal or placental complication. The result must be considered alongside maternal symptoms, gestational age, fetal movement, and any other relevant clinical findings.

For example, a patient undergoing routine surveillance at 36 weeks because of gestational diabetes may have a baseline fetal heart rate of 145 bpm, moderate variability, and two qualifying accelerations during the first 20 minutes. If the remaining tracing and clinical assessment are reassuring, the NST would generally be classified as reactive.

What if accelerations do not occur within 20 minutes?

The absence of two qualifying accelerations during the initial 20 minutes does not automatically mean that the NST is abnormal. The fetus may be in a sleep cycle, and fetal activity can vary throughout the observation period.

The healthcare provider may continue monitoring, commonly extending the observation period to approximately 40 minutes in total, to allow time for spontaneous fetal movement and associated heart rate accelerations. In selected circumstances, a clinician may use an appropriate stimulation technique according to local protocol.

If the required accelerations remain absent after the assessment period, the result may be classified as nonreactive. This distinction is important because a nonreactive NST describes what was observed during the test; it does not independently establish the cause or confirm fetal compromise. 

A practical approach to interpreting accelerations

When assessing accelerations, consider the following:

1. Establish the baseline fetal heart rate before measuring the increase.
2. Determine how far the heart rate rises above baseline.
3. Measure how long the increase lasts.
4. Apply the criteria appropriate for the gestational age.
5. Count the qualifying accelerations within the required monitoring period.
6. Review the tracing as a whole and correlate the changes with recorded fetal movement when possible.

A heart rate increase that is too brief or too small to meet the applicable criteria should not automatically be counted as a qualifying acceleration. Similarly, the absence of a recorded movement marker does not necessarily mean that the fetus did not move.

Decelerations and Nonreactive NST Results

Decelerations are temporary decreases in the fetal heart rate below the established baseline. They are important because their timing, duration, depth, and relationship to uterine contractions can help clinicians determine whether further assessment is needed.

Unlike accelerations, which are generally reassuring when they meet the required criteria, decelerations require interpretation according to their characteristics and the circumstances in which they occur. A deceleration does not automatically mean that the fetus is experiencing fetal distress, but persistent or concerning patterns may indicate a need for prompt clinical evaluation.

The interpretation of decelerations during an NST differs somewhat from interpretation during labor. During labor, clinicians assess heart rate changes in relation to contractions and the progression of labor. During an NST, the primary purpose is antepartum fetal surveillance, although uterine activity may also be recorded. 

Types of decelerations and their clinical significance

Several deceleration patterns are recognized in fetal heart rate monitoring.

• Early decelerations: These are gradual decreases in heart rate that typically occur in association with uterine contractions, with the lowest point of the deceleration corresponding approximately to the peak of the contraction. They are classically associated with fetal head compression during labor. They are less central to routine NST interpretation when labor is not occurring.
• Late decelerations: These are gradual decreases in heart rate that begin after a contraction starts, reach their lowest point after the contraction peaks, and recover after the contraction ends. During labor, recurrent late decelerations may indicate impaired fetal oxygenation associated with uteroplacental insufficiency. Their significance depends on the wider clinical picture.
• Variable decelerations: These are abrupt decreases in heart rate that vary in timing, duration, or appearance. They are commonly associated with umbilical cord compression and may occur with or without uterine contractions. Their clinical significance depends on their frequency, depth, duration, recovery, and associated features.
• Prolonged decelerations: These are decreases of at least 15 bpm below baseline lasting at least two minutes but less than 10 minutes. They warrant assessment because they may reflect a significant temporary change in fetal status. A decrease lasting 10 minutes or longer is classified as a baseline change rather than a prolonged deceleration under standard terminology.

These definitions provide a framework for recognizing heart rate patterns, but they should not be used to diagnose fetal compromise without clinical assessment. For example, a brief variable deceleration may have a different significance from recurrent decelerations accompanied by absent variability.

Understanding a nonreactive NST

A nonreactive NST is one in which the required number of qualifying accelerations is not observed within the applicable testing period. For a fetus at 32 weeks or later, this generally means that fewer than two qualifying accelerations occur during the usual assessment period, even after additional monitoring when indicated.

Several factors may explain a nonreactive result:

• Fetal sleep cycles: The fetus may be temporarily less active, resulting in fewer observable accelerations.
• Earlier gestational age: The fetal nervous system is still developing, so the expected heart rate response may be less pronounced.
• Maternal medications: Certain medications can influence fetal activity or heart rate patterns.
• Fetal or placental conditions: In some cases, the absence of expected accelerations may reflect a problem that requires additional evaluation.
• Technical limitations: Poor signal quality or difficulty identifying the baseline can interfere with interpretation.

A nonreactive result should therefore prompt consideration of both physiological and clinical explanations. It is not equivalent to a diagnosis of fetal distress, and it does not automatically mean that immediate delivery is necessary.

For example, a patient at 30 weeks of gestation may undergo an NST because of hypertension. If the tracing shows a stable baseline but no qualifying accelerations during the initial monitoring period, the result may reflect developmental factors or a fetal sleep cycle. The healthcare provider may extend the monitoring period and consider further assessment based on the complete clinical picture.

If the NST remains nonreactive, additional testing may include a biophysical profile, which combines the NST with ultrasound assessment of fetal activity and amniotic fluid, or other investigations appropriate to the suspected problem. The next step depends on gestational age, maternal condition, fetal movement, and the overall findings rather than on the nonreactive label alone. 

How to interpret concerning decelerations

When decelerations occur, the clinician evaluates more than the lowest heart rate recorded. Important features include:

1. Frequency: Whether decelerations occur occasionally or repeatedly.
2. Duration: How long the fetal heart rate remains below baseline.
3. Depth: How far the heart rate falls below the baseline.
4. Recovery: Whether the heart rate returns promptly to baseline.
5. Associated variability: Whether variability remains moderate or becomes minimal or absent.
6. Clinical context: Whether there are uterine contractions, reduced fetal movement, maternal illness, or other findings that increase concern.

A tracing with recurrent concerning decelerations, particularly when accompanied by absent variability or sustained bradycardia, requires prompt assessment by the appropriate clinical team. The response should follow local obstetric protocols and account for the maternal and fetal condition.

For nursing practice, accurate documentation and communication are essential. The nurse should ensure that the fetal monitor is recording a reliable signal, identify significant changes, document the relevant observations, and promptly report concerning findings to the responsible clinician. A normal baseline should not be used to dismiss recurrent decelerations or other abnormal features.

In summary, learning how to read an NST requires interpreting the baseline, variability, accelerations, and decelerations together. A reactive NST is generally reassuring at the time of testing, while a nonreactive result indicates that further assessment may be needed. The most appropriate interpretation always depends on gestational age, maternal and fetal circumstances, tracing quality, and the full clinical picture.

NST Procedure and Fetal Activity Monitoring

The nonstress test (NST) is performed by recording the fetal heart rate over time and examining its relationship to fetal activity. The procedure is noninvasive and does not ordinarily require medication, needles, or deliberate induction of uterine contractions. It is commonly performed in an antenatal clinic, maternity assessment unit, or hospital when additional information about fetal well-being is needed.

During the test, the healthcare professional observes the fetal heart rate tracing for a reassuring baseline, appropriate accelerations, and any concerning changes. The monitoring equipment may also record uterine activity, helping the clinician identify contractions occurring during the assessment. These observations are interpreted in relation to gestational age, maternal symptoms, the reason for testing, and the overall clinical picture.

Fetal Movement and Its Relationship to Heart Rate Accelerations

Fetal movement is an important part of NST interpretation because movement is often accompanied by a temporary increase in the fetal heart rate. These increases, called accelerations, provide information about the responsiveness of the fetal nervous and cardiovascular systems. A normal response is generally reassuring, although the absence of an acceleration during a particular movement does not automatically mean that the fetus is compromised.

The relationship between fetal movement and heart rate changes depends partly on the development of the fetal autonomic nervous system. As gestation progresses, the mechanisms that regulate the fetal heart rate mature, making the expected acceleration pattern more consistent. This is one reason why the criteria used to interpret an NST differ before and after 32 weeks of gestation. 

How fetal movement produces heart rate accelerations

When a fetus moves, signals involving the nervous system can temporarily increase the heart rate. The fetal monitor records these changes, allowing the healthcare professional to determine whether the response meets the criteria for an acceleration.

For a fetus at 32 weeks of gestation or later, the standard acceleration criteria generally require an increase of at least 15 beats per minute above baseline lasting at least 15 seconds. Before 32 weeks, an increase of at least 10 beats per minute lasting at least 10 seconds is commonly used. A reactive NST at or beyond 32 weeks typically requires at least two qualifying accelerations within a 20-minute period. 

For example, consider a fetus with a baseline heart rate of 140 beats per minute. During fetal movement, the heart rate rises to 158 beats per minute for 20 seconds before returning to baseline. This represents an acceleration of 18 beats per minute lasting 20 seconds. If the fetus is at least 32 weeks of gestation, the acceleration meets the usual amplitude and duration criteria.

The healthcare professional then assesses the complete tracing to determine whether sufficient qualifying accelerations occurred during the monitoring period. The number of movements reported by the pregnant person is useful context, but the NST is interpreted primarily from the recorded heart rate pattern.

Fetal sleep cycles and reduced activity during testing

Fetal movement is not continuous. Fetuses alternate between periods of activity and relative quiet, and a sleep cycle may temporarily reduce movement and the number of associated accelerations. As a result, a healthy fetus may not demonstrate the required heart rate response during the first 20 minutes of monitoring.

When the tracing is otherwise suitable for continued observation but lacks sufficient accelerations, the healthcare professional may extend the NST to approximately 40 minutes to allow for a change in the fetal behavioral state. Depending on local protocols and clinical circumstances, vibroacoustic stimulation may also be considered to encourage fetal activity. 

For example, a patient at 34 weeks of gestation may report that the baby usually moves frequently in the evening, but the NST takes place in the morning when activity is lower. If the initial tracing does not show enough accelerations, the clinician may continue monitoring rather than immediately concluding that the result indicates fetal compromise.

However, reduced movement reported by the pregnant person should not simply be attributed to sleep. A noticeable decrease from the fetus’s usual movement pattern warrants prompt contact with the maternity care team, even if a previous NST was reassuring. The need for assessment is determined by the current symptoms and clinical situation.

Why fetal movement matters even when the NST is reactive

A reactive NST indicates a reassuring heart rate response during the test, but it is not a guarantee of continuing fetal well-being. Fetal activity can change after monitoring ends, and the NST does not directly measure every aspect of fetal health.

For this reason, fetal movement assessment and electronic monitoring complement one another. A movement concern may prompt an NST, while the NST findings help determine whether additional evaluation is appropriate. If decreased fetal movement persists or recurs, the pregnant person should seek clinical advice rather than relying on an earlier reassuring result.

NST Preparation, Equipment, and Test Duration

An NST generally requires minimal preparation because it is a noninvasive procedure. The purpose of preparation is to ensure that the pregnant person is comfortable, the fetal monitor obtains a reliable signal, and the tracing provides enough information for accurate interpretation.

1. Preparing for the NST

Before starting the test, the healthcare professional confirms the reason for monitoring and reviews relevant information, including gestational age, maternal medical conditions, medications, fetal movement concerns, and previous NST results.

Preparation commonly involves the following steps:

1. Explain the procedure: Describe how the NST records the fetal heart rate and why monitoring is being performed. Explain that the test does not ordinarily cause pain or deliberately induce contractions.
2. Position the pregnant person comfortably: A reclining or semi-recumbent position is commonly used. A lateral position may be preferred when needed for comfort or to avoid lying flat on the back for a prolonged period.
3. Expose the abdomen appropriately: The abdominal skin is made accessible so the monitoring sensors can be positioned correctly.
4. Apply the sensors: The healthcare professional places the fetal heart rate transducer over the abdomen where the fetal heartbeat can be detected clearly and secures it with an elastic belt.
5. Check signal quality: The fetal heart rate display and tracing are reviewed to ensure that the monitor is recording consistently.
6. Explain movement recording: The pregnant person may be asked to press a marker button whenever fetal movement is felt, allowing the event to be compared with the heart rate tracing.

These steps help establish a reliable record while keeping the patient comfortable. Routine fasting is generally unnecessary, although individual instructions may vary according to the facility or the reason for the visit. 

2. Equipment used during an NST

The primary equipment consists of an electronic fetal monitor and external sensors attached to the abdomen.

Fetal heart rate transducer

This sensor commonly uses Doppler ultrasound to detect fetal cardiac activity and produce a continuous record of the fetal heart rate. Its position may need adjustment if the signal becomes weak or the fetus changes position.

Uterine activity sensor

An external tocodynamometer, or TOCO sensor, detects changes in abdominal wall tension associated with uterine contractions. It helps identify the timing and approximate duration of contractions but does not reliably measure their absolute strength.

Electronic monitor and tracing

The monitor displays or prints the fetal heart rate pattern and, when the uterine activity sensor is used, a corresponding record of uterine activity. A movement marker may also be recorded to help correlate perceived movement with heart rate changes.

An important technical consideration is that the fetal heart rate transducer can occasionally lose contact or record an unreliable signal. Maternal movement, fetal position, and sensor displacement may interrupt the tracing. The healthcare professional should reposition the sensor and verify the signal rather than interpret a gap or artifact as a genuine change in fetal heart rate.

It is also important to distinguish the fetal heart rate from the maternal pulse when the tracing is unclear. If the displayed rate appears inconsistent with the clinical findings, the healthcare professional should verify the source of the signal according to local monitoring procedures.

3. How long does an NST take?

An NST typically begins with at least 20 minutes of fetal heart rate monitoring. If the required accelerations occur and the tracing is adequately reassuring, the test may be completed after that period. If the result remains nonreactive, monitoring may be extended to approximately 40 minutes to account for fetal sleep cycles. Additional assessment may be necessary if the tracing remains nonreactive or other findings are concerning. 

The duration can vary according to several factors:

• Whether the fetus demonstrates the required accelerations early in the test.
• Whether the fetus appears to be in a sleep cycle.
• Whether the fetal monitor obtains a consistent, interpretable signal.
• Whether decelerations or other concerning heart rate patterns require assessment.
• Whether the clinical circumstances indicate that additional testing is needed.

For example, a patient undergoing routine antepartum fetal surveillance may have a reactive NST within 20 minutes and finish the procedure shortly afterward. Another patient may require 40 minutes of monitoring because the fetus is initially quiet. If the second tracing remains nonreactive, the healthcare provider decides whether a biophysical profile or another form of fetal assessment is indicated.

The test should not be ended solely because a predetermined time has elapsed if the tracing is inadequate or concerning. Similarly, a nonreactive result should not be treated as proof of fetal compromise without appropriate clinical evaluation.

Uterine Contractions During an NST

Uterine contractions are periods when the uterine muscle tightens and then relaxes. They can occur during pregnancy even when labor has not begun. During an NST, a contraction may be detected by the uterine activity sensor, allowing the healthcare professional to compare its timing with changes in the fetal heart rate.

The NST is designed to assess fetal heart rate responses under ordinary conditions rather than deliberately stress the fetus. Uterine contractions are not required for an NST to be considered reactive. The primary assessment remains the fetal heart rate pattern and the presence of the required accelerations. 

How contractions are recorded during an NST

When a TOCO sensor is used, it detects changes in tension at the abdominal surface. A contraction typically appears as a rise on the uterine activity tracing followed by a return toward baseline as the uterus relaxes.

The recording can help the clinician identify:

• When contractions begin and end.
• How frequently contractions occur.
• Whether fetal heart rate changes coincide with contractions.
• Whether uterine activity is accompanied by findings that require further assessment.

However, an external TOCO sensor does not directly measure the pressure generated inside the uterus. Its recorded amplitude can be affected by sensor placement, abdominal tissue, and belt tension. A taller waveform does not necessarily mean that the contraction is stronger than another contraction.

Interpreting fetal heart rate changes during contractions

When a contraction occurs, the healthcare professional observes whether the fetal heart rate remains stable, accelerates, or decreases. The timing and nature of any decrease help determine its significance.

For example, occasional uterine tightening may appear on the tracing without any concerning fetal heart rate change. This does not, by itself, indicate a problem. Conversely, a deceleration that repeatedly occurs in association with contractions, particularly when accompanied by other concerning features, requires clinical assessment.

The distinction between an NST and a contraction stress test is important here. During an NST, contractions are observed if they occur naturally; they are not deliberately induced to evaluate the fetal response. A contraction stress test, by contrast, evaluates fetal heart rate responses in relation to uterine contractions that are deliberately produced under clinical supervision when the test is indicated. 

When uterine activity requires further attention

Contractions during an NST may warrant additional assessment when they are frequent, painful, associated with vaginal bleeding or fluid leakage, or accompanied by concerning fetal heart rate changes. The significance depends on gestational age and the patient’s symptoms.

For example, if a patient at 32 weeks experiences regular painful contractions during monitoring, the healthcare professional should assess the symptoms and determine whether evaluation for preterm labor is necessary. The NST alone cannot establish whether labor is progressing; this may require further obstetric assessment.

Similarly, if contractions coincide with recurrent decelerations, the clinical team should assess the complete tracing and the maternal and fetal condition. The response should follow the appropriate obstetric protocol rather than relying on the contraction waveform alone.

In summary, uterine activity provides useful context during an NST, but it is not the primary measure of reactivity. Understanding how the fetal heart rate, fetal movement, and contractions are recorded together helps clinicians interpret the tracing accurately and recognize when additional assessment may be needed.

NST vs. Contraction Stress Test and Biophysical Profile

Antepartum fetal surveillance involves several tests designed to assess fetal well-being before labor begins. The nonstress test (NST), contraction stress test (CST), and biophysical profile (BPP) are related assessments, but they evaluate different aspects of fetal health. Understanding their differences helps healthcare professionals interpret findings appropriately and recognize when additional testing may be necessary.

The NST primarily evaluates the fetal heart rate response to spontaneous fetal movement. The contraction stress test assesses how the fetal heart rate responds to uterine contractions, while the biophysical profile combines fetal heart rate monitoring with ultrasound observations of fetal activity and amniotic fluid volume. These tests may be used individually or in combination, depending on the pregnancy, maternal health, gestational age, and initial findings. 

No single test provides a complete assessment of every aspect of fetal health. A reactive NST is generally reassuring at the time of testing, but it does not directly measure fetal growth or amniotic fluid volume. Similarly, a normal amniotic fluid measurement does not independently establish that every aspect of fetal well-being is satisfactory. The healthcare provider interprets each result within the broader clinical picture.

Nonstress Test vs. Contraction Stress Test

The nonstress test and contraction stress test both use fetal heart rate monitoring, but the principal difference is whether uterine contractions are required for the assessment.

The NST observes the fetal heart rate during a period of spontaneous fetal activity and rest. The contraction stress test evaluates the fetal heart rate response during uterine contractions, which are produced under clinical supervision when indicated. The latter examines whether contractions are associated with particular heart rate changes that may suggest difficulty tolerating the temporary reduction in uteroplacental blood flow that can occur during a contraction.

1. Purpose and principles of the NST

The NST is commonly used for antepartum fetal surveillance when a pregnancy has a condition that warrants additional monitoring or when there is concern about fetal movement.

During the test, an external fetal monitor records the heart rate, allowing the healthcare professional to assess the baseline, variability, accelerations, and any decelerations. For a fetus at 32 weeks of gestation or later, a reactive NST generally requires at least two accelerations of 15 beats per minute or more above baseline, each lasting at least 15 seconds, within a 20-minute period.

The principal advantage of the NST is that it is noninvasive and does not deliberately induce contractions. It can provide useful information about fetal heart rate reactivity without exposing the patient to the additional uterine activity associated with a contraction stress test.

For example, a patient at 35 weeks of gestation with gestational diabetes may undergo an NST as part of a planned surveillance schedule. If the tracing shows a normal baseline, moderate variability, and the required accelerations without concerning findings, the result is generally reassuring. Further management depends on the overall clinical situation and the surveillance plan.

2. Purpose and principles of the contraction stress test

The contraction stress test assesses fetal heart rate responses during uterine contractions. Its underlying principle is that contractions temporarily reduce blood flow through the placenta, and a fetus with limited placental reserve may demonstrate concerning heart rate changes during this period.

During the test, uterine contractions are monitored alongside the fetal heart rate. If spontaneous contractions are insufficient for the assessment, contractions may be induced using a clinician-directed method, such as carefully supervised oxytocin administration or nipple stimulation, according to the applicable protocol. The test requires close monitoring because inducing contractions may not be appropriate for every pregnancy.

The results are generally interpreted as follows:

• Negative CST: No late decelerations are observed with the required pattern of contractions. This is generally a reassuring result.
• Positive CST: Late decelerations occur with at least half of the contractions in an adequate contraction pattern. This is a concerning result that requires clinical interpretation and management.
• Equivocal or suspicious CST: The heart rate findings do not clearly meet the criteria for a negative or positive result, or the pattern is concerning but inconsistent.
• Unsatisfactory CST: The test cannot be interpreted adequately, for example, because there are insufficient contractions or the fetal heart rate tracing is inadequate.

A positive contraction stress test does not independently establish fetal distress, but it raises concern that the fetus may not tolerate contractions well. The healthcare provider considers gestational age, maternal condition, the tracing, and other investigations when deciding on further surveillance or delivery.

3. Key differences between the two tests

The choice between an NST and a contraction stress test depends on the clinical question being addressed. An NST is often used as an initial surveillance test, whereas a contraction stress test may be considered in selected situations when further information about the fetal response to contractions is needed. It is not automatically the next test after every nonreactive NST; a biophysical profile or another assessment may be more appropriate.

Example of clinical application

Consider a patient at 37 weeks of gestation who is undergoing fetal surveillance because of a maternal medical condition. The NST does not demonstrate the required accelerations, even after an appropriate monitoring period. This is a nonreactive result, but it does not automatically mean that the fetus is compromised.

The healthcare provider may recommend a biophysical profile to assess fetal movement, tone, breathing movements, and amniotic fluid volume. A contraction stress test may be considered in selected circumstances, but the decision depends on the clinical situation, whether the test is appropriate, and the information still needed. The purpose is to clarify fetal status rather than to assume that one abnormal screening result confirms a problem.

Biophysical Profile and Amniotic Fluid Assessment

A biophysical profile (BPP) is a prenatal assessment that combines fetal heart rate monitoring with ultrasound observations to evaluate several indicators of fetal well-being. It is commonly used when an NST is nonreactive, when other findings raise concern, or when a pregnancy requires a more comprehensive assessment of fetal status.

Unlike an NST, which focuses primarily on the fetal heart rate response to movement, the biophysical profile examines fetal behavior and the amount of amniotic fluid surrounding the fetus. These additional observations provide information that cannot be obtained from the heart rate tracing alone. 

Components of the biophysical profile

A complete BPP evaluates five components, with each assigned a score of either 0 or 2 points. The maximum total is 10 points.

Each component is scored according to specific criteria and the applicable testing protocol. A score of 2 indicates that the criteria for that component have been met, while a score of 0 indicates that they have not been met. The score should be interpreted by a qualified healthcare professional in relation to gestational age and the complete clinical picture.

A modified BPP uses a different combination of assessments: the NST and an ultrasound assessment of amniotic fluid volume. It is less extensive than the complete BPP but can be useful for surveillance in appropriately selected pregnancies. 

Understanding BPP scores

The total BPP score helps guide clinical interpretation and follow-up. Commonly used interpretations are:

• 8–10 points: Generally reassuring when amniotic fluid volume is adequate.
• 6 points: An equivocal result that may require repeat testing, additional assessment, or consideration of delivery, depending on gestational age and clinical circumstances.
• 4 points or lower: Concerning and generally requires further evaluation and consideration of more urgent management.

These scores are not used in isolation. Amniotic fluid volume is particularly important: low fluid can warrant closer surveillance or consideration of delivery even when the overall score might otherwise appear reassuring. 

For example, a patient at 36 weeks of gestation has a nonreactive NST. A complete BPP may be performed to assess fetal breathing movements, body movements, muscle tone, and amniotic fluid volume. If the total score is 8 out of 10 and the fluid assessment is normal, the findings may be reassuring, depending on which component received zero points and the wider clinical situation. If the score is lower or fluid volume is reduced, further evaluation may be necessary.

A nonreactive NST should therefore be understood as one finding within a broader assessment process. The BPP provides additional information that may help distinguish a temporarily quiet fetus from a situation requiring closer surveillance.

Amniotic fluid assessment and its importance

Amniotic fluid surrounds the fetus, cushions it against external pressure, allows movement, and supports normal musculoskeletal development. It also contributes to the environment required for fetal lung development. The amount of fluid is therefore an important part of ultrasound-based fetal assessment.

Amniotic fluid volume changes throughout pregnancy and is influenced by several factors, including fetal urine production, swallowing, membrane integrity, and placental and maternal conditions. Ultrasound assessment can help identify fluid volumes that are lower or higher than expected.

Two important abnormalities are:

• Oligohydramnios: Lower-than-expected amniotic fluid volume. It may be associated with ruptured membranes, placental insufficiency, fetal urinary tract abnormalities, or other pregnancy complications.
• Polyhydramnios: Excessive amniotic fluid. It can be associated with maternal diabetes, certain fetal conditions, or may occur without an identified cause.

Low amniotic fluid may be significant because it can occur in pregnancies affected by placental dysfunction or reduced fetal urine production. However, it does not independently confirm that the fetus is currently experiencing oxygen deprivation. The healthcare provider considers the fluid measurement alongside fetal growth, maternal history, gestational age, membrane status, and other test results.

How amniotic fluid is measured

Ultrasound assessment commonly uses one of two approaches:

1. Amniotic fluid index (AFI): The uterus is divided into four sections, and the vertical depth of the largest suitable fluid pocket in each section is measured. The four measurements are added to produce the AFI.
2. Single deepest vertical pocket: The deepest suitable pocket of fluid is measured vertically, avoiding fetal parts and the umbilical cord.

The method used depends on the clinical situation and the relevant protocol. These measurements help clinicians identify abnormal fluid volume and decide whether additional monitoring or intervention may be needed. 

For example, a patient with hypertension and suspected fetal growth restriction may have an NST that appears reactive but an ultrasound showing reduced amniotic fluid. Although the NST is reassuring at the time of testing, the low fluid finding may indicate a need for closer surveillance or consideration of delivery. This illustrates why fetal heart rate findings and ultrasound assessments should be interpreted together.

Applying the findings in clinical practice

The NST, contraction stress test, biophysical profile, and amniotic fluid assessment serve complementary purposes. The appropriate test depends on the clinical question, gestational age, and the findings already available.

When interpreting these assessments, healthcare professionals should consider:

• Whether the fetal heart rate tracing meets the criteria for a reactive NST.
• Whether ultrasound observations show expected fetal movement, tone, and breathing movements.
• Whether amniotic fluid volume is adequate.
• Whether maternal conditions or fetal growth concerns increase the need for surveillance.
• Whether the combined findings support routine follow-up, additional testing, or timely intervention.

A reassuring test result provides useful information about fetal status at the time of assessment, but it cannot guarantee that complications will not develop later. Conversely, an abnormal result does not always mean that immediate delivery is necessary. The appropriate response depends on the overall clinical picture and the balance between the risks of continuing pregnancy and those associated with early delivery. ACOG emphasizes individualized decisions when planning antenatal fetal surveillance and responding to its findings.

Factors Affecting NST Results and Clinical Follow-Up

Interpreting a nonstress test (NST) requires more than identifying whether the result is reactive or nonreactive. The meaning of the tracing depends on gestational age, fetal activity, maternal health, the quality of the fetal heart rate signal, and the reason for antepartum fetal surveillance. A result that requires further assessment does not automatically mean that the fetus is experiencing fetal distress; however, it should never be dismissed without considering the full clinical picture.

When learning how to read an NST, it is important to distinguish between a genuinely concerning fetal heart rate pattern and a result affected by fetal sleep, prematurity, medication, or technical limitations. The healthcare provider combines NST results with maternal symptoms, the pregnancy history, other fetal assessment findings, and gestational age to determine whether continued monitoring or additional testing is appropriate. The American College of Obstetricians and Gynecologists (ACOG) emphasizes that the timing and frequency of antepartum fetal surveillance should be individualized according to the risks and circumstances of each pregnancy.

Effects of Gestational Age, Fetal Sleep, and Maternal Factors

Gestational age is one of the most important factors affecting NST results. Fetal nervous system development influences how the fetal heart rate responds to movement. Earlier in pregnancy, the autonomic nervous system is less mature, so the fetus may not demonstrate the same acceleration patterns expected later in gestation. Consequently, a nonreactive NST in a preterm fetus does not have the same significance as a similar result in a fetus at a later gestational age.

At or after 32 weeks of gestation, a commonly used criterion for a reactive NST is at least two fetal heart rate accelerations of 15 beats per minute or more above baseline, each lasting at least 15 seconds, within a 20-minute period. Before 32 weeks, clinicians may use the more appropriate 10-beat-per-minute rise lasting at least 10 seconds, depending on the clinical protocol. A normal preterm fetus may not meet the later-pregnancy criteria because its heart rate patterns are still developing. ACOG notes that nonreactive findings are relatively common in normal fetuses between 24 and 28 weeks and remain possible between 28 and 32 weeks. 

For example, if an NST is performed at 29 weeks and the fetus shows movement but does not produce two accelerations meeting the 15-by-15 criterion, the healthcare provider should consider gestational age before interpreting the result. The provider may use an age-appropriate criterion, extend the monitoring period, or arrange additional fetal assessment if indicated. The result should not be classified as fetal compromise solely because the criteria used for a later gestation have not been met.

Fetal sleep cycles can also produce a nonreactive NST. Fetuses alternate between periods of activity and rest. During a quiet sleep cycle, the fetus may move less frequently and show fewer heart rate accelerations, even when fetal well-being is normal. A brief 20-minute recording may therefore fail to capture enough accelerations to meet the criteria for a reactive result.

When the tracing is otherwise appropriate and there is no urgent clinical concern, monitoring may be extended, often up to 40 minutes, to allow time for the fetus to become active. A clinician may also use an appropriate stimulation technique, such as vibroacoustic stimulation, to encourage fetal activity. The response is then assessed by observing whether the fetal heart rate accelerates. A nonreactive result caused by sleep is different from a persistent lack of reactivity accompanied by other concerning findings, which may require further investigation. 

Maternal and pregnancy-related factors can influence fetal activity or the interpretation of an NST:

• Medications: Certain medications can reduce fetal activity or alter fetal heart rate patterns. The healthcare provider should review relevant prescription medicines and other substances before interpreting a nonreactive NST. Medication use should not be stopped or changed without clinical advice.
• Maternal health conditions: Hypertension, preeclampsia, and diabetes may increase the risk of placental dysfunction or other pregnancy complications. In these circumstances, the reason for ordering the NST matters as much as the tracing itself.
• Maternal symptoms: Decreased fetal movement, vaginal bleeding, abdominal pain, or other new symptoms may change the urgency of fetal assessment, even if a previous NST was reactive.
• Fetal growth and placental function: Suspected fetal growth restriction can indicate an increased risk of placental insufficiency. An NST may be combined with ultrasound and umbilical artery Doppler assessment to obtain a broader picture of fetal health.
• Gestational age near or beyond term: When a pregnancy continues to 41 weeks or later, antepartum fetal surveillance may be recommended because risks change as gestation advances. The timing and frequency of testing depend on the clinical situation and local guidance.

These factors do not independently prove that an NST is abnormal or that the fetus is not receiving enough oxygen. Instead, they help determine how much confidence to place in the result and whether additional assessment is necessary. A reactive NST can be reassuring at the time of testing, but it cannot guarantee fetal well-being for the remainder of pregnancy or replace evaluation of new symptoms

Technical Limitations of Fetal Heart Rate Monitoring

The accuracy of NST interpretation depends on the quality of the recorded signal. A fetal monitor typically uses an external Doppler ultrasound transducer to detect the fetal heart rate. If the transducer shifts, contact is poor, or the fetus changes position, the monitor may lose the signal or record an inconsistent heart rate. These interruptions can make it difficult to identify the baseline fetal heart rate, variability, accelerations, and decelerations reliably.

A tracing with frequent signal loss may not provide enough continuous information to classify an NST confidently as reactive or nonreactive. In such cases, the healthcare professional may reposition the transducer, adjust the belts, assess the maternal pulse, and continue monitoring until an adequate recording is obtained. The quality of the tracing should be evaluated before drawing conclusions from the apparent heart rate patterns.

Maternal heart rate artifact is an important source of potential error. An external fetal monitor can sometimes detect the pregnant person’s heart rate rather than the fetal heart rate. Because maternal and fetal heart rates can occasionally overlap, the tracing may appear plausible even though it does not accurately represent the fetus. This can create false reassurance if a concerning fetal pattern is masked, or lead to unnecessary intervention if the maternal signal is mistaken for an abnormal fetal rate. Guidance on fetal heart rate surveillance therefore emphasizes awareness of maternal heart rate artifact and confirmation of the fetal signal when there is doubt. 

For example, if the monitor suddenly displays a rate that closely matches the maternal pulse, or the tracing changes unexpectedly when the pregnant person moves, the clinician should check signal quality rather than automatically interpreting the change as a fetal acceleration or deceleration. Depending on the setting and available equipment, the team may compare the maternal pulse with the monitor, reposition the transducer, or use ultrasound to confirm the fetal heart rate.

Other limitations should also be considered:

• Fetal position and movement: A change in fetal position can weaken the Doppler signal and create gaps in the tracing. Repositioning the transducer may be needed to obtain a clearer recording.
• Maternal movement or body habitus: Movement and difficulty maintaining transducer contact can affect signal quality. These factors may make monitoring more challenging but do not, by themselves, indicate a problem with fetal health.
• Multiple gestation: In a twin or higher-order pregnancy, the monitor must capture and distinguish the heart rate of each fetus. The same fetal signal should not be mistaken for two separate heart rates.
• Limited information from external monitoring: An NST records heart rate patterns over a particular period. It does not directly measure fetal oxygen levels, diagnose every cause of fetal compromise, or independently establish placental function or amniotic fluid volume.
• Interpretation in context: A brief change in heart rate should not be assessed in isolation. The baseline, variability, accelerations, decelerations, signal quality, gestational age, and clinical circumstances all contribute to interpretation.

These limitations explain why learning how to read an NST involves assessing both the pattern and the reliability of the recording. If the tracing is technically inadequate, the appropriate response is to improve or verify the signal and obtain sufficient information—not to label the test reassuring or abnormal without adequate evidence. When the tracing is technically adequate but remains nonreactive or shows concerning features, the clinician considers further fetal assessment.

Follow-Up Testing and Clinical Management of Abnormal NST Results

An abnormal or nonreactive NST requires clinical interpretation and, when appropriate, further fetal assessment. A nonreactive NST is not a diagnosis of fetal distress; it means that the required heart rate accelerations were not observed during the testing period. The reason may be temporary, such as fetal sleep or gestational age, but it may also indicate that additional information is needed to assess fetal well-being. The next step depends on the complete tracing, gestational age, maternal condition, fetal movement, and the original indication for testing. 

Common follow-up steps include:

1. Reviewing and extending the NST. If the fetus may be sleeping and the tracing is otherwise reassuring, monitoring can be extended to allow additional time for movement and accelerations. An appropriate stimulation technique may also be considered. The clinician reviews whether the fetal heart rate meets the criteria for a reactive NST after the longer observation period.

2. Performing a biophysical profile (BPP). A BPP combines NST findings with ultrasound assessment of fetal breathing movements, gross body movements, muscle tone, and amniotic fluid volume. This provides additional information when an NST is nonreactive or when the overall clinical situation requires a more comprehensive assessment. A modified BPP combines the NST with an ultrasound assessment of amniotic fluid.

3. Assessing amniotic fluid and fetal growth. Ultrasound may be used to evaluate amniotic fluid volume and fetal growth when placental dysfunction, growth restriction, or another pregnancy complication is suspected. Low amniotic fluid can require closer surveillance or consideration of delivery, even when other findings appear reassuring.

4. Considering a contraction stress test (CST). In selected cases, a CST may be used to evaluate how the fetal heart rate responds to uterine contractions. Because contractions place additional demands on placental oxygen transfer, this test can provide information that is not obtained from an NST alone. It is not appropriate for every pregnancy and requires clinical assessment before use.

5. Increasing surveillance or considering delivery. If additional tests suggest that the fetus may be at risk, the healthcare team may recommend more frequent monitoring, hospital assessment, treatment of a maternal condition, or delivery. The decision depends on the severity and persistence of the findings, gestational age, maternal health, and the anticipated benefits and risks of continuing the pregnancy.

These approaches are not a mandatory sequence for every nonreactive result. Some cases require only a longer recording, while others need prompt assessment. ACOG emphasizes that abnormal antenatal fetal surveillance results must be interpreted alongside the overall maternal and fetal condition and gestational age. Because false-positive results occur, an abnormal NST—particularly early in pregnancy—does not automatically mean that delivery is necessary. 

Clinical example: Consider a pregnancy at 35 weeks in which the fetus does not demonstrate the required accelerations during the initial 20-minute NST. If fetal movement is limited and the tracing suggests a sleep cycle, the clinician may extend the recording. If the NST remains nonreactive, a BPP or another appropriate assessment may be recommended. If the additional testing is reassuring, continued surveillance may be suitable. If it reveals concerning findings, such as significantly reduced amniotic fluid or persistent abnormalities in fetal heart rate patterns, the healthcare team must assess whether closer monitoring or delivery is safer than continuing the pregnancy.

A different approach may be needed when a nonreactive NST occurs alongside decreased fetal movement, persistent decelerations, or a serious maternal complication. These findings warrant prompt clinical assessment rather than assuming the fetus is asleep. A previously reactive NST should not delay evaluation of a new concern about fetal movement or maternal health.

For pregnancies complicated by hypertension, gestational diabetes, suspected growth restriction, or other high-risk conditions, follow-up plans should reflect the underlying risk as well as the NST results. Depending on the condition, antepartum fetal surveillance may be repeated weekly or more frequently. ACOG notes that the optimal timing and frequency of testing are not established for every condition, so the plan should be individualized rather than applied uniformly to all high-risk pregnancies. 

Accurate documentation and communication are also essential. Clinical records should identify the gestational age, reason for testing, duration and adequacy of the recording, baseline fetal heart rate, variability, accelerations and decelerations, final interpretation, and any follow-up plan. When the tracing is concerning or technically inadequate, the responsible clinician should be informed promptly according to the facility’s escalation protocol.

For anyone learning how to read an NST, the central principle is that the result must be interpreted in context. A reactive NST is generally reassuring for the period observed, while a nonreactive NST indicates that further interpretation or testing may be needed. Neither result should be treated as a complete assessment of fetal health on its own. Combining fetal heart rate monitoring with clinical history, maternal assessment, and additional prenatal testing supports more informed decisions about continued surveillance and the timing of intervention.

Conclusion

Understanding how to read an NST is essential for interpreting fetal heart rate patterns and assessing fetal well-being during pregnancy. By evaluating the baseline fetal heart rate, variability, accelerations, and decelerations, healthcare professionals can determine whether the findings are reassuring or whether further assessment may be necessary. A reactive nonstress test generally indicates a reassuring fetal heart rate response at the time of monitoring, while a nonreactive result calls for careful evaluation rather than an immediate assumption of fetal distress.

Accurate NST interpretation also requires consideration of gestational age, fetal sleep cycles, maternal health conditions, fetal movement, and the technical quality of the recording. When results are inconclusive or concerning, additional assessments, such as a biophysical profile, ultrasound evaluation of amniotic fluid, or a contraction stress test in selected cases, may provide valuable information for clinical decision-making. Combining these findings with the patient’s history and current clinical condition helps guide appropriate follow-up and individualized prenatal care.

For nurses and other healthcare professionals, proficiency in reading an NST extends beyond recognizing normal and abnormal patterns. It involves accurate documentation, timely communication of concerning findings, and appropriate escalation of care according to clinical protocols. Most importantly, an NST should never be interpreted in isolation or treated as a guarantee of continuing fetal well-being. Recognizing its purpose, limitations, and role in broader antepartum fetal surveillance supports informed clinical decisions, timely intervention when indicated, and safer outcomes for both the pregnant patient and the fetus.

Frequently Asked Questions

1. How do you interpret NST results?

A nonstress test (NST) is interpreted by assessing the fetal heart rate baseline, variability, accelerations, and decelerations. A reactive NST is generally reassuring and, at 32 weeks of gestation or later, typically shows at least two accelerations of 15 beats per minute above baseline lasting at least 15 seconds within 20 minutes. Before 32 weeks, the 10-by-10 criteria may be used. A nonreactive NST lacks sufficient accelerations within the observation period and may require extended monitoring or additional testing.

2. What does a failed NST look like?

A “failed NST” commonly refers to a nonreactive result. The tracing does not show enough qualifying fetal heart rate accelerations during the monitoring period. It may also contain concerning features, such as recurrent decelerations or abnormal variability. However, a nonreactive NST does not automatically mean fetal distress; fetal sleep, gestational age, and other factors can affect the result. Further assessment may be necessary.

3. How to read an NST machine?

An NST machine typically displays two main tracings:

• Upper tracing: Fetal heart rate, measured in beats per minute (bpm). Look at the baseline, variability, accelerations, and decelerations.
• Lower tracing: Uterine activity, showing the timing and pattern of contractions or tightening.

The monitoring period commonly lasts about 20–40 minutes, although it may be extended if the fetus is sleeping or the findings are inconclusive. Interpret the tracing alongside gestational age and the patient’s clinical condition.

4. What do the numbers on an NST mean?

The numbers depend on the machine and display, but commonly include:

• Fetal heart rate (bpm): The normal baseline range is generally 110–160 bpm.
• Acceleration: A temporary rise in fetal heart rate; at 32 weeks or later, a rise of at least 15 bpm lasting at least 15 seconds generally meets the standard criterion.
• Variability: Small fluctuations in the fetal heart rate that reflect changes in the fetal autonomic nervous system.
• Uterine activity: Indicates the timing and relative pattern of contractions; external monitoring does not reliably measure their strength in absolute pressure units.

Important: NST results should be interpreted by a qualified healthcare professional. Persistent abnormal patterns, recurrent decelerations, or other concerning findings require prompt clinical assessment.