Nonstress test

The nonstress test (NST) is a noninvasive diagnostic procedure used in prenatal care to assess fetal well-being by monitoring the fetal heart rate (FHR) in response to the fetus's own movements over a specified period, without applying any external stimuli to the fetus.^[1]^[2] Primarily employed in high-risk pregnancies—such as those involving gestational diabetes, intrauterine growth restriction, preeclampsia, or post-term gestation—the NST helps detect potential fetal distress and reduce the risk of stillbirth by evaluating the fetus's autonomic nervous system function through heart rate accelerations.^[1]^[2] It is typically initiated around 32 to 34 weeks of gestation, though earlier use may occur in certain high-risk cases, and is often performed weekly or twice weekly depending on the clinical context.^[1]^[2] During the test, the pregnant individual lies in a semi-Fowler or left lateral position while external transducers are placed on the abdomen to record the FHR and uterine activity for at least 20 minutes, extending to 40 minutes if needed; fetal movements may be noted manually or via the monitor, and vibroacoustic stimulation can sometimes be applied to provoke responses if activity is low.^[1]^[2] Results are classified as reactive (reassuring) if there are at least two FHR accelerations of 15 beats per minute or more above the baseline, lasting at least 15 seconds each, within a 20-minute window, indicating healthy fetal oxygenation and neurological integrity; nonreactive results, which may arise from fetal sleep or other benign factors, necessitate further evaluation such as a biophysical profile or contraction stress test.^[1]^[2] The NST carries no significant risks, as it is painless and radiation-free, though it has limitations including a potential for false positives leading to unnecessary interventions or false negatives in acute distress scenarios like placental abruption.^[1]^[2]

Background

Definition

A nonstress test (NST), also known as a fetal nonstress test, is a noninvasive screening tool used in prenatal care to evaluate fetal well-being by monitoring the fetal heart rate (FHR) and its accelerations in response to spontaneous fetal movements, without the induction of uterine contractions.^[1] The test assesses the fetus's ability to respond to its own activity, providing insight into oxygenation and neurological integrity, and is typically performed after 32 weeks of gestation, as fetal heart rate patterns become more reliable thereafter.^[3] The key components of an NST involve external fetal monitoring devices strapped to the maternal abdomen. A cardiotocograph, which uses Doppler ultrasound, continuously records the FHR, while fetal movements are often noted by maternal perception, palpation, or occasionally a tocodynamometer to detect associated uterine changes, though the latter is not always required for the basic test.^[2] This setup allows for real-time tracing of FHR patterns without internal probes or pharmacological intervention, emphasizing the test's simplicity and safety.^[4] Developed in the 1970s, the NST emerged as a straightforward alternative to more invasive methods like the oxytocin challenge test, which induced contractions to stress the fetus and observe FHR responses. Pioneering work by researchers such as Lee et al. and Rochard et al. in the mid-1970s established clinical protocols for NST, demonstrating its efficacy in antepartum surveillance and reducing the need for riskier procedures.^[5] At its core, the NST is grounded in fetal physiology, relying on the autonomic nervous system's reactivity: in a healthy fetus, movements trigger parasympathetic-mediated accelerations in FHR, typically defined as an increase of at least 15 beats per minute above the baseline for a minimum of 15 seconds, signaling adequate fetal reserve.^[6] This response reflects intact cardioacceleratory pathways and is a key indicator of normal fetal development.

Purpose and Indications

The nonstress test (NST) serves as a primary tool for antenatal fetal surveillance, aimed at evaluating fetal oxygenation and overall well-being by monitoring fetal heart rate (FHR) patterns in response to fetal movements. This non-invasive assessment helps detect early signs of fetal hypoxia or uteroplacental insufficiency, which could indicate compromised fetal status, thereby reducing the risk of stillbirth in high-risk pregnancies.^[3]^[2] NST is indicated for various high-risk pregnancy conditions where the stillbirth risk exceeds 0.8 per 1,000 with a relative risk greater than 2.0 compared to low-risk pregnancies. Common indications include gestational or pregestational diabetes, hypertensive disorders such as chronic hypertension or preeclampsia, post-term pregnancy beyond 41 weeks gestation, intrauterine growth restriction, oligohydramnios, and decreased fetal movement. Additional scenarios encompass multiple gestations (particularly monochorionic twins), previous stillbirth at or after 32 weeks, and conditions like Rh incompatibility or maternal clotting disorders that elevate fetal compromise risk.^[3]^[2]^[7] Typically, NST is initiated at or by 32 weeks of gestation for most high-risk cases, though earlier initiation may occur upon diagnosis of severe conditions or fetal viability. In high-risk scenarios, testing frequency is generally weekly, increasing to twice weekly for conditions with rapid deterioration potential, such as preeclampsia with severe features or poorly controlled pregestational diabetes. The overarching goal of NST is to provide a safe, outpatient screening method that informs decisions on escalating to more invasive testing, such as biophysical profiles, or prompting timely delivery to safeguard fetal health.^[3]^[2]

Procedure

Patient Preparation

Prior to undergoing a nonstress test (NST), patients receive an explanation of the procedure's purpose, expected sensations, and noninvasive nature to facilitate informed consent and shared decision-making.^[2] Dietary guidance typically encourages consumption of a light snack or sugary drink shortly before the test to promote fetal activity, as this can help elicit heart rate accelerations; no fasting is required, and patients are often advised to empty their bladder for comfort.^[4]^[7] Patients are positioned in a semi-Fowler's or left lateral decubitus position on a reclining chair or exam table to minimize aortocaval compression by the gravid uterus, thereby optimizing maternal hemodynamics and fetal oxygenation.^[2]^[4]^[7] For equipment setup, two elastic belts are secured around the abdomen after applying ultrasound gel to the skin: one holds a Doppler ultrasound transducer to monitor fetal heart rate, while the other features a tocodynamometer to detect uterine contractions or fetal movements.^[2]^[4]^[7] The baseline test duration is planned for 20 to 40 minutes to capture adequate fetal heart rate patterns, though it may extend up to 40 minutes if initial results are nonreactive to confirm reactivity.^[2]^[1]

Test Administration

The nonstress test (NST) is administered in a clinical setting using external electronic fetal monitoring to continuously record the fetal heart rate (FHR) and detect fetal movements, typically for a minimum of 20 minutes or until two qualifying accelerations are observed.^[8] The patient is positioned in a semi-Fowler or left lateral position to optimize blood flow and comfort, with two transducers secured to the abdomen via elastic belts: an ultrasound transducer over the fetal back to capture FHR and a pressure transducer to monitor any spontaneous uterine contractions.^[2] This noninvasive approach requires no internal devices, such as a fetal scalp electrode, distinguishing it from intrapartum monitoring methods.^[9] The test is conducted in an outpatient antenatal testing unit or hospital labor and delivery area, allowing for routine surveillance without hospitalization in most cases.^[8] An obstetrician, certified nurse-midwife, or trained perinatal nurse supervises the procedure, observing the monitor in real time for FHR patterns and patient-reported or marker-indicated fetal movements.^[4] If fetal activity remains low after 20 minutes, the provider may apply vibroacoustic stimulation—a brief sound and vibration device placed on the maternal abdomen—to arouse the fetus and potentially elicit movements and accelerations, extending the monitoring period up to 40 minutes if needed.^[8] For fetuses at or beyond 32 weeks of gestation, a qualifying acceleration is defined as an abrupt increase in FHR of at least 15 beats per minute above the baseline, sustained for at least 15 seconds, with two such events within the initial 20-minute window indicating reactivity (standard criteria for NST); for preterm fetuses before 32 weeks, the criteria are at least 10 beats per minute above baseline lasting at least 10 seconds.^[8]^[2] The normal FHR baseline during the test ranges from 110 to 160 beats per minute.^[8] Documentation consists of a real-time strip chart printout or digital electronic tracing that captures the FHR baseline, moderate variability, accelerations, and any noted fetal movements, providing a permanent record for clinical review.^[10]

Interpretation

Normal and Abnormal Patterns

A nonstress test (NST) is interpreted primarily based on the fetal heart rate (FHR) response to fetal movement, focusing on accelerations, variability, and the absence of concerning decelerations.^[8] A reactive, or normal, result is defined as at least two FHR accelerations, each rising at least 15 beats per minute (bpm) above the baseline and lasting at least 15 seconds, occurring within a 20-minute observation period, typically associated with fetal movement.^[3] This pattern indicates intact fetal autonomic nervous system function and adequate oxygenation, reassuring fetal well-being.^[2] In contrast, a nonreactive result occurs when the required accelerations are absent over an extended monitoring period, often up to 40 minutes, despite the presence of fetal movement.^[8] While a nonreactive NST may reflect benign states such as fetal sleep or preterm gestation, it can also signal potential issues like hypoxia, prompting the need for extended monitoring or additional diagnostic tests.^[2] Beyond accelerations, baseline FHR variability plays a key role in interpretation; moderate variability, with an amplitude range of 6-25 bpm, is considered normal and reassuring, reflecting a healthy fetal central nervous and cardiovascular system.^[3] Absent or minimal variability (undetectable or ≤5 bpm) is abnormal and concerning, as it may indicate fetal compromise, while marked variability (>25 bpm) is less common but can occur in response to stimuli.^[2] Decelerations observed during an NST are also evaluated; early decelerations, mirroring uterine contractions, are typically benign and related to head compression.^[2] Variable decelerations, characterized by abrupt drops of at least 15 bpm lasting 15 seconds to 2 minutes, may suggest umbilical cord compression if recurrent (≥3 in 20 minutes) or prolonged (≥2 minutes).^[2] Late decelerations, with a gradual onset and nadir after the contraction peak, are abnormal and indicative of uteroplacental insufficiency, warranting immediate further assessment.^[3] FHR tracings from NSTs are often classified using the National Institute of Child Health and Human Development (NICHD) three-tier system for standardized interpretation.^[11] Category I (normal) includes a baseline FHR of 110-160 bpm, moderate variability, presence of accelerations, and absence of late or variable decelerations, predicting no fetal acidosis.^[11] Category II (indeterminate) encompasses tracings not meeting Category I or III criteria, such as minimal variability or recurrent variable decelerations without progression, requiring close surveillance.^[11] Category III (abnormal) features absent variability with recurrent late, variable, or prolonged decelerations, or bradycardia, indicating potential fetal compromise and necessitating prompt intervention.^[11] The International Federation of Gynecology and Obstetrics (FIGO) guidelines align closely, emphasizing similar features for reassuring (normal), suspicious (indeterminate), and pathological (abnormal) tracings.^[11] For clinical management, a reactive NST (Category I) generally permits discharge or continuation of routine antenatal surveillance without immediate concern.^[8] Nonreactive results or Category II/III tracings prompt further evaluation, such as a biophysical profile or contraction stress test, to confirm fetal status and guide decisions on delivery if indicated.^[3]

Influencing Factors

Several maternal factors can influence the outcomes of a nonstress test (NST). Gestational age plays a significant role, particularly before 32 weeks, when fetal neurologic immaturity often results in fewer heart rate accelerations and a higher incidence of nonreactive tests, necessitating cautious interpretation. Maternal medications, such as beta-blockers like propranolol, can reduce fetal heart rate variability and acceleration frequency by lowering the baseline heart rate. Additionally, nicotine exposure from cigarette smoking decreases fetal heart rate reactivity and variability, potentially leading to nonreactive results, while caffeine consumption may enhance fetal activity and improve test reactivity. Fetal factors also affect NST results. The fetal sleep-wake cycle is a common cause of nonreactivity, as the fetus may spend up to 40 minutes in quiet sleep, during which accelerations are absent; extending the test duration accounts for this physiological state. In multiple gestations, such as twins, tracing interpretation is complicated by overlapping signals and higher rates of nonreactivity, especially in preterm cases. Fetal anomalies, including anemia from conditions like Rh isoimmunization, can produce abnormal patterns such as reduced variability or decelerations, serving as predictors of compromised fetal status. Technical issues during NST administration can obscure accurate readings. Improper belt or transducer placement may fail to capture fetal heart rate signals effectively, while maternal obesity can attenuate ultrasound transmission, leading to poor-quality tracings and the need for repositioning or alternative monitoring techniques. To mitigate these influencing factors, clinicians often prolong the NST beyond the standard 20-40 minutes, encourage maternal hydration to potentially increase fetal movement, or apply vibroacoustic or light stimulation to arouse the fetus from sleep and elicit accelerations. If initial results are nonreactive, repeat testing is recommended to distinguish benign causes from true pathology. Nonreactive NSTs carry a high false-positive rate of 50-75% for fetal hypoxia, meaning most such results do not indicate actual compromise but rather transient influences like sleep or medications, underscoring the importance of confirmatory tests.

Clinical Applications

Diagnostic Utility

The nonstress test (NST) demonstrates high diagnostic utility in antepartum fetal surveillance, particularly through its strong negative predictive value. A reactive NST, characterized by at least two accelerations of the fetal heart rate in 20-40 minutes, has a negative predictive value of approximately 99% for fetal well-being, indicating a low likelihood of fetal acidosis or demise in the subsequent week.^[8] Conversely, a nonreactive result exhibits lower positive predictive value, typically ranging from 20-50%, meaning it is less reliable for confirming adverse outcomes and often requires confirmatory testing.^[12] This asymmetry underscores the NST's primary strength in ruling out compromise rather than confirming it, with sensitivity around 50% and specificity exceeding 90% in various studies.^[13] Supported by authoritative guidelines, the NST plays a key role in reducing perinatal mortality among high-risk pregnancies. The American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin No. 229 recommends NST as a first-line method for antepartum surveillance in conditions such as gestational diabetes, hypertension, and post-term pregnancy, typically starting at 32-34 weeks gestation.^[14] Evidence from randomized trials and cohort studies integrated into these guidelines shows that routine NST use can decrease intrauterine fetal death rates by identifying at-risk fetuses early, though benefits are most pronounced when combined with clinical judgment.^[14] A reactive NST specifically correlates with the absence of fetal acidosis, as accelerations reflect intact neurological and cardiovascular responses to movement, providing reassurance without invasive measures.^[15] The NST is often employed alongside the amniotic fluid index to enhance its predictive accuracy, forming the modified biophysical profile for comprehensive assessment.^[14] This combination improves detection of oligohydramnios-related risks while maintaining the test's non-invasive nature. The NST has affirmed utility in high-risk scenarios, including pregnancies complicated by COVID-19, where weekly NSTs help monitor potential placental insufficiency.^[16] Additionally, the NST's cost-effectiveness stems from its brevity—typically under 30 minutes—and lack of radiation or instrumentation, making it accessible for outpatient settings and reducing overall healthcare expenditures compared to more complex alternatives.^[5]

Limitations and Alternatives

The nonstress test (NST) has several limitations that can affect its clinical utility. It exhibits a high false-positive rate, often resulting in nonreactive tracings that do not indicate true fetal compromise, which may lead to unnecessary interventions such as preterm delivery or additional testing.^[3] Interpretation of NST results is operator-dependent, with significant interobserver and intraobserver variability that can influence reliability and lead to inconsistent assessments.^[17] The test is not reliable before 24-28 weeks of gestation, as nonreactivity is common due to fetal immaturity—up to 50% at 24 weeks and 15% at 28-32 weeks—rather than distress, necessitating caution in preterm pregnancies.^[18] Additionally, the NST primarily detects acute fetal stress and may miss chronic hypoxia without accompanying acute events, limiting its ability to identify ongoing placental insufficiency.^[2] Risks associated with the NST are minimal, involving only mild discomfort from the placement of monitoring belts on the abdomen, with no direct risks to the fetus. However, false-positive results can induce maternal anxiety from perceived alarms or the need for follow-up procedures.^[4]^[19] Alternatives to the NST include the biophysical profile (BPP), which integrates the NST with ultrasound evaluation of fetal breathing movements, body movements, tone, and amniotic fluid volume to provide a more comprehensive assessment.^[2] The contraction stress test (CST) induces uterine contractions to evaluate fetal heart rate response, offering higher sensitivity for detecting uteroplacental insufficiency but is more invasive and contraindicated in cases of preterm labor or placental previa.^[2] Umbilical artery Doppler velocimetry serves as another option, particularly for suspected fetal growth restriction, by measuring blood flow resistance in the umbilical artery to identify chronic hypoxia.^[2] Alternatives are selected based on clinical context; a nonreactive NST typically prompts progression to a full BPP for further evaluation. In equivocal cases, the modified biophysical profile—combining the NST with amniotic fluid index assessment—balances efficiency and diagnostic depth without full ultrasound components.^[1]^[2] Recent developments as of 2024 include FDA-cleared remote fetal monitoring devices, such as the INVU platform by Nuvo, enabling self-operated NST assessments at home to reduce in-person visits in high-risk pregnancies, supported by state Medicaid reimbursement policies in several U.S. states.^[20]^[21]