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Nonstress test and contraction stress test

Nonstress test and contraction stress test
Literature review current through: May 2024.
This topic last updated: Jul 28, 2023.

INTRODUCTION — Fetal health is evaluated, in part, by assessment of fetal heart rate patterns. The primary goal of antepartum fetal surveillance (antepartum testing) with the nonstress test (NST) and the contraction stress test (CST) is to identify fetuses at risk of hypoxic injury or death and intervene to prevent these adverse outcomes, if possible. The secondary goal is to identify normally oxygenated fetuses so that pregnancy can be continued safely, and unnecessary intervention can be avoided.

An abnormal test (nonreactive NST, positive CST) is sometimes associated with adverse fetal or neonatal outcomes, while a normal test (reactive NST, negative CST) is usually associated with a neurologically intact and adequately oxygenated fetus. When interpreting these tests, the clinician needs to account for gestational age, results of prior fetal assessment, maternal conditions (including medications), and fetal condition (eg, growth restriction, anemia, arrhythmia).

The NST and CST will be reviewed here. Intrapartum fetal evaluation and additional techniques for assessing fetal health are discussed separately.

(See "Intrapartum fetal heart rate monitoring: Overview".)

(See "Biophysical profile test for antepartum fetal assessment".)

(See "Decreased fetal movement: Diagnosis, evaluation, and management".)

(See "Doppler ultrasound of the umbilical artery for fetal surveillance in singleton pregnancies".)

PHYSIOLOGIC BASIS OF FETAL HEART RATE CHANGES — Physiologic development of the fetal heart occurs across gestation and affects fetal heart rate (FHR) patterns. Changes in FHR result from moment-to-moment autonomic modulation in response to many factors, including input from chemoreceptors, input from baroreceptors, central nervous system activities (eg, arousal, sleep), catecholamines, and blood volume [1].

Effect of gestational age on fetal heart rate — The parasympathetic and sympathetic nervous systems exert a progressively greater influence on the FHR as gestational age advances. Parasympathetic innervation of the heart is mediated primarily by the vagus nerve, which influences the sinoatrial (SA) and atrioventricular (AV) nodes. Parasympathetic stimulation slows the FHR, and blockade by parasympatholytic medications (eg, atropine) increases FHR [2]. Sympathetic stimulation of the heart increases the FHR, and blockade of sympathetic activity slows the FHR.

With advancing gestational age, the maturation of the parasympathetic system causes slowing of the baseline heart rate but usually not below the normal range of 110 to 160 beats per minute (figure 1). Maturation of the sympathetic system causes an increase in the frequency and amplitude of FHR accelerations [3,4]. For example, 50 percent of normal fetuses demonstrated accelerations with fetal movements at 24 weeks, while over 95 percent demonstrated accelerations at 30 weeks of gestation, in one study [5]. Before 32 weeks, accelerations may increase by only 10 beats per minute above the baseline and last 10 seconds, whereas later in gestation, accelerations of 15 beats per minute above the baseline and lasting 15 seconds are expected [6].

Cardiovascular response to hypoxemia — Fetal oxygenation depends upon the adequate transfer of oxygen from the environment to the fetal tissues. Oxygen is transferred from the environment to fetal tissues by maternal and fetal blood along a pathway that includes the maternal lungs, heart, vasculature, and uterus and the fetal placenta and umbilical cord. Fetal hypoxemia (usually expressed as the partial pressure of oxygen dissolved in blood, or PO2) can result from interruption of the transfer of oxygen from the environment to fetal tissue at any point along this pathway.

The FHR response to interrupted oxygenation depends on the cause:

Transient fetal hypoxemia associated with uterine contractions can cause late decelerations. Stimulation of chemoreceptors in the fetal carotid arteries and aortic arch leads to a reflex increase in sympathetic outflow, causing vasoconstriction in nonvital peripheral areas and preservation of, or increases in, blood flow to vital organs (eg, adrenal glands, heart, brain). Peripheral vasoconstriction causes elevated fetal blood pressure and, in turn, stimulation of baroreceptors in the fetal carotid arteries and aortic arch, resulting in reflex, vagally mediated, parasympathetic slowing of FHR shortly after the beginning of the contraction.

Transient interruption of fetal oxygenation caused by compression of the umbilical cord can result in variable decelerations. If cord compression reduces blood flow in the umbilical vein first, transient hypovolemia may cause a reflex rise in FHR (sometimes referred to as a "shoulder"). Subsequent arterial compression increases blood pressure, leading to vagally mediated slowing of FHR until the cord compression resolves.

Acute ongoing interruption of fetal oxygenation at the level of the maternal lungs (eg, maternal hypoxemia), maternal heart (eg, acute reduction in cardiac output), maternal vasculature (eg, maternal hypotension), uterus (eg, tetanic contraction, uterine rupture), placenta (eg, abruption), or umbilical cord (eg, prolapse) can cause prolonged decelerations.

By comparison, early decelerations are not caused by fetal hypoxemia and not associated with adverse outcome. They are caused by fetal head compression, which increases intracranial pressure, resulting in reflex slowing of the FHR. Available evidence does not support fetal extracranial pressure exerted by uterine contractions as a cause of fetal brain injury [7].

EQUIPMENT AND NOMENCLATURE

Equipment — Electronic fetal monitors are used to continuously record the fetal heart rate (FHR) in graphical form. In most external FHR monitoring systems, the FHR is measured by focusing an ultrasound beam on the fetal heart from a small Doppler ultrasound device placed on the maternal abdomen. A bedside monitor interprets the Doppler signals, which reflect cardiac motion. The complex wave generated is analyzed, and the peak is used for calculations. An internal computer then calculates the FHR by averaging several consecutive peak-to-peak frequencies to minimize artifact. This process of averaging is called "autocorrelation." It produces a FHR waveform closely resembling that derived from a fetal electrocardiogram (ECG).

A recent generation of fetal monitors calculates FHR, FHR variability, and fetal QT intervals from information obtained from multiple electrodes placed on the maternal abdomen (eg, Monica AN24 monitor, MindChild Medical monitor). The maternal ECG signal and ambient noise are filtered out by algorithms run by microprocessors. This software provides noninvasive fetal ECGs that are purported to be as accurate as the information obtained from a fetal scalp electrode [8-10]. It is useful in patients with obesity in whom continuous Doppler ultrasound monitoring is technically difficult and does not provide an adequate quality FHR tracing for interpretation.

Nomenclature — The most commonly used American system for describing FHR findings was developed by a workshop convened by the National Institute of Child Health and Human Development (table 1) [11].

OVERVIEW OF ANTEPARTUM FETAL HEART RATE TESTING

Background — The NST and CST are used for antepartum fetal heart rate (FHR) testing. Although one study reported a lower incidence of fetal death among high-risk pregnancies undergoing antepartum testing (with NST and amniotic fluid index [AFI]) compared with low-risk untested pregnancies, there remains no conclusive evidence from randomized trials that use of NSTs and CSTs leads to a reduction in fetal death or neurologic injury [12]. Nevertheless, performing NSTs or CSTs in pregnancies deemed to be at high risk of adverse fetal outcome is a standard obstetric practice in the United States and elsewhere, based on circumstantial evidence and long-standing convention. (See 'Test performance' below.)

Indications — The primary goal of antepartum testing is to reduce the risk of stillbirth by identifying fetuses at risk for hypoxic injury or death [13]. A secondary goal is to identify normally oxygenated fetuses so that expectant management of high-risk pregnancies can be continued safely. Thus, antepartum FHR testing is performed in pregnancies considered to be at increased risk of fetal hypoxic injury or demise. Examples include pregnancies that are complicated by maternal conditions associated with placental dysfunction or poor uteroplacental perfusion. Some fetal disorders are also associated with an increased risk of fetal hypoxic injury or death. (See "Overview of antepartum fetal assessment", section on 'Indications for fetal assessment'.)

Test performance — Use of the NST and CST became widespread in the early 1980s when a seminal observational study reported stillbirth rates (corrected for lethal congenital anomalies and unpredictable causes of demise) after reactive and nonreactive NSTs were 1.9 and 26 per 1000 births, respectively [14]. The stillbirth rates after a negative CST, reactive positive CST, or nonreactive positive CST were 0.3, 0, and 88 per 1000 births, respectively. These findings supported use of these tests to identify fetuses who might benefit from early delivery.

In 2015, a Cochrane review attempted to determine whether using the NST can improve maternal or perinatal outcome by identifying high-risk pregnancies requiring prompt induction of labor or immediate delivery by cesarean [15]. Six randomized trials involving 2105 participants were included. Tested patients were compared with controls who did not undergo NSTs or their test results were concealed. No significant differences between groups were noted in maternal outcomes, such as frequency of induction of labor or cesarean birth, or in infant outcomes, including perinatal mortality, low five-minute Apgar, neonatal intensive care unit admission, and neonatal seizures. Perinatal mortality (adjusted for lethal anomalies) in the testing group was higher than in controls (2.3 versus 1.1 percent, four studies, n = 1627, odds ratio 2.05, 95% CI 0.95-4.42). By contrast, when the authors compared computerized cardiotocography (automated evaluation of the FHR) versus traditional cardiotocography, computerized cardiotocography was associated with a statistically significant reduction in perinatal mortality (relative risk [RR] 0.20, 95% CI 0.04-0.88, two trials, 0.9 versus 4.2 percent, n = 469). There was no statistically significant difference in potentially preventable deaths (RR 0.23, 95% CI 0.04-1.29, two trials, n = 469); however, the meta-analysis was underpowered to assess this outcome.

There are many limitations to the trials in this meta-analysis, which preclude definitive assessment of the value of the NST in current practice. Randomization methods were suboptimal, which could have created bias. The total number of adverse events, such as perinatal mortality and abnormal neurologic signs, was too low to determine whether small, but statistically significant, differences in outcome might result from testing. Furthermore, the trials were conducted in the early 1980s when these tests were first introduced into clinical practice and clinicians were inexperienced in test interpretation and subsequent pregnancy management. It is possible that the tests would be interpreted and acted upon differently today with different outcomes. Lastly, fetal assessment and neonatal care have changed since the 1980s; the combined use of ultrasound and FHR testing may be more predictive of fetal status and need for intervention than either test alone.

Choice of test — There is no evidence from randomized trials on which to base a recommendation for use of the CST versus the NST for antepartum fetal monitoring. Although some observational studies suggest that the CST is more predictive of adverse outcome than the NST [14,16], others have not demonstrated any improvement in predicting perinatal morbidity compared with the NST, even when both tests are combined [17]. The only randomized trial comparing antepartum fetal testing methods was inconclusive [18]. This trial compared the biophysical profile (BPP; which includes an NST) with the NST alone in management of 652 high-risk pregnancies. Costs were not considered.

However, the CST is less convenient to perform than the NST since it requires either an intravenous line for infusion of a dilute oxytocin solution or nipple stimulation, and it tends to be more time consuming. It also has some relative contraindications. (See 'Overview' below.)

Timing and frequency — NSTs and CSTs are initiated at the gestational age that the fetus is believed to be at increased risk of death as long as the age is sufficiently advanced that delivery for perinatal benefit would be considered. For the NST, fetal neurologic maturity should be sufficient to enable FHR acceleration (typically no earlier than 26 to 28 weeks).

There is no high-quality evidence defining the optimal frequency of testing [19]. Reported test frequencies include weekly NST, weekly CST with midweek NST, twice weekly NST with once weekly AFI, and twice weekly NST with twice weekly AFI [20]. Testing is performed periodically as long as the indication for testing persists, but maternal or fetal deterioration requires reevaluation despite recent normal test results [13].

Maternal or fetal deterioration requires reevaluation despite recent normal test results. Although a pattern of accelerations, moderate variability, and no significant decelerations during the NST and CST reliably identifies the absence of ongoing hypoxic injury during the test, it does not preclude such injury in the future, especially if there are significant changes in the clinical setting over time.

Management of pregnancies with normal or abnormal test results — An abnormal test result is generally followed by additional testing with a different test, given the high rate of false-positive results. The clinical setting also needs to be considered. If a temporary maternal condition may account for the abnormal test result, prompt treatment of the maternal condition may also improve fetal oxygenation and lead to a normal test result on subsequent testing. In chronic conditions, clinical judgment guides management and needs to consider case-specific factors. (See "Overview of antepartum fetal assessment", section on 'Follow-up of pregnancies with normal test results' and "Overview of antepartum fetal assessment", section on 'Management of pregnancies with abnormal test results'.)

NONSTRESS TEST

Overview — The NST is the most common cardiotocographic method of antepartum fetal assessment. It is noninvasive and can be performed in any setting where an electronic fetal monitor is available. There is no direct risk of maternal or fetal injury associated with NSTs.

The NST may be performed with or without sonographic assessment of amniotic fluid volume. When using amniotic fluid volume assessment, the deepest vertical pocket of fluid, rather than the amniotic fluid index (AFI), may be associated with fewer unnecessary interventions without an increase in adverse perinatal outcomes [13]; however, not all experts agree and further study is warranted [21]. (See "Overview of antepartum fetal assessment", section on 'Biophysical profile and modified biophysical profile'.)

Reactive tests

Criteria — The NST is reactive from 32 weeks to term if there are two or more fetal heart rate (FHR) accelerations reaching a peak of at least 15 beats per minute (bpm) above the baseline rate and lasting at least 15 seconds from onset to return to baseline (15 x 15) in a 20-minute period (figure 2 and table 1) [6,13,22]. A reactive test provides reliable evidence of normal fetal oxygenation, regardless of the length of observation time needed to demonstrate reactivity [23].

Before 32 weeks of gestation — Different criteria have been suggested for gestational ages less than 32 weeks (see 'Effect of gestational age on fetal heart rate' above). Before 32 weeks of gestation, a reactive NST may be defined as two accelerations that rise at least 10 bpm above baseline and have a duration of at least 10 seconds (10 x 10) [11,13]. Whether clinicians choose to apply the conventional 15 x 15 criteria or adopt the 10 x 10 criteria when performing NSTs before 32 weeks of gestation, they should use the same approach in all patients. However, once a fetus has demonstrated the maturity to generate accelerations of 15 bpm for 15 seconds, an acceleration of 10 bpm for 10 seconds may not have the same ability to confirm normal fetal oxygenation, even before 32 weeks of gestation. Available evidence has not resolved this issue definitively.

The validity of the 10 x 10 criteria before 32 weeks was based on the following two studies:

In one study of 143 participants who underwent antepartum testing before 32 weeks, the frequency of adverse perinatal outcomes was similar when 10 x 10 criteria were used to define a normal test rather than the conventional 15 x 15 criteria, and the time required to achieve a reactive NST was shorter with the less stringent criteria [24]. However, the study was not powered to detect differences in adverse outcome or to confirm the safety of applying the 10 x 10 criteria before 32 weeks.

In another study including 488 subjects undergoing antepartum testing before 32 weeks, perinatal outcome was similar whether the last NST before delivery was reactive using 10 x 10 or 15 x 15 criteria [25]. However, after controlling for known confounding factors, such as gestational age and birth weight, there was a trend toward more perinatal deaths following a reactive 10 x 10 NST than following a reactive 15 x 15 NST.

Minimal duration of FHR monitoring — The optimal duration of the NST has not been established. Some sources recommend the NST should be continued for at least 20 minutes, even if two qualifying accelerations have been observed before that time [26]. However, large studies evaluating the predictive value of the NST combined with amniotic fluid volume assessment have not included this requirement [12,27,28].

Reactive NSTs with decelerations — The significance of a reactive NST with FHR decelerations is unclear. Multiple observational studies have described an increased frequency of intrapartum FHR decelerations and operative delivery when this combination occurs [29-34], except when the decelerations are brief [35]; however, outcomes are usually good. The majority of FHR decelerations during the NST are variable decelerations, reflecting transient episodes of umbilical cord compression.

Variable, late, or prolonged decelerations observed during antepartum testing require further evaluation, which might include extended FHR and uterine activity monitoring, ultrasound assessment of fetal growth and anatomy, BPP, amniotic fluid volume, and/or Doppler velocimetry in the setting of fetal growth restriction. Management decisions should be guided by the results of the additional evaluation and other details specific to the clinical situation. (See "Doppler ultrasound of the umbilical artery for fetal surveillance in singleton pregnancies" and "Fetal growth restriction: Pregnancy management and outcome", section on 'Fetal surveillance'.)

Intermittent fetal cardiac arrhythmias can also cause decelerations and may be diagnosed by echocardiography. Management depends on the arrhythmia and patient-specific factors. (See "Fetal arrhythmias".)

Nonreactive tests

Criteria and causes — A NST is nonreactive if it does not meet acceleration criteria for a reactive NST (see 'Reactive tests' above). The FHR should be monitored for at least 40 minutes before interpreting the test as nonreactive.

Nonreactivity may be a sign of interrupted fetal oxygenation to the point of metabolic acidemia. The mean umbilical vein pH associated with a nonreactive NST is 7.28±0.11, which is higher than the pH associated with a low biophysical profile (BPP) score, 7.16±0.08 [36]. (The normal range of antepartum fetal umbilical vein pH established by cordocentesis is shown in the table (table 2).)

Other possible causes of a nonreactive NST include fetal immaturity, fetal sleep, maternal smoking, fetal neurologic or cardiac anomalies, sepsis, or maternal ingestion of drugs with cardiac effects [37]. Sleep is a common and benign cause of a nonreactive NST. Sleep cycles may last up to 40 minutes [5]; one study reported sleep cycles lasting up to 53 minutes [38]. In a study that observed late preterm fetuses from uncomplicated pregnancies for 100 minutes, quiet sleep occurred at least once in 30 percent of fetuses, but 96 percent of the fetuses cycled between quiet sleep and active states during the period of observation [39].

Evaluation of pregnancies with nonreactive NSTs — Up to 60 percent of nonreactive NSTs may be false positives, usually defined as a nonreactive NST that prompts delivery, but delivery is not associated with findings suggestive of acute interruption of fetal oxygenation (eg, FHR decelerations or loss of variability, meconium passage, operative delivery for acute FHR changes, low Apgar scores, or abnormal umbilical cord blood gas values) or chronic suboptimal fetal oxygenation (eg, fetal growth restriction, oligohydramnios).

A nonreactive NST usually warrants further evaluation. Some options include:

Repeat the test in 30 minutes.

Perform vibroacoustic stimulation to elicit accelerations.

Perform a back-up test, (either CST or complete BPP) [13]. (See "Biophysical profile test for antepartum fetal assessment".)

If possible, modify factors potentially causing nonreactive results (eg, smoking proximate to the test).

Vibroacoustic stimulation can decrease the number of nonreactive NSTs and shorten test time without reducing the predictive value of a reactive NST. A vibroacoustic source, typically an artificial larynx, placed on or just above the maternal abdomen, is used to stimulate fetal movement. In a 2013 systematic review, vibroacoustic stimulation decreased mean overall testing time by almost seven minutes, compared with mock or no stimulation (-6.93, 95% CI -12.09 to -1.76), and reduced the frequency of nonreactive NSTs by 40 percent (odds ratio 0.62, 95% CI 0.48-0.81) [40]. There are no evidence-based standards for performing this procedure. It has been performed as soon as five minutes after initiation of the NST. The stimulus is applied for one to five seconds and may be repeated. The optimal placement and number of applications of the stimulus have not been evaluated. The American College of Obstetricians and Gynecologists suggests positioning the device on the maternal abdomen and applying a stimulus for one to two seconds [13]. If no fetal response occurs, the stimulus may be repeated up to three times for progressively longer durations of up to three seconds.

Transabdominal light stimulation with a halogen light for 10 seconds appears to stimulate the fetus and may be as effective as vibroacoustic stimulation [41-43]. More safety and efficacy data are needed before this method can be recommended in place of vibroacoustic stimulation. One study found that fetal exposure to recorded music was associated with increased fetal movement, increased number of accelerations, and improved variability [44]. Musical intervention may be a noninvasive and inexpensive tool for shortening the time to reactivity.

Although commonly practiced, neither maternal glucose administration nor transabdominal manual fetal manipulation significantly decreases the incidence of nonreactive test results [45,46].

Changing maternal position does not increase reactivity as long as the patient is tested in a position that does not lead to hypotension from uterine compression of the great vessels [47]. Cocoa and caffeine consumption may affect fetal movement, but the dose, timing, and effect on NST reactivity have not been evaluated [48-50]. No randomized trials or observational studies have assessed the effect of maternal hydration (oral or intravenous) on FHR reactivity. Maternal hydration (oral or intravenous) may increase the AFI [51-53] and decrease the baseline FHR [54,55], but there is no evidence that it increases fetal movement or heart rate reactivity.

CONTRACTION STRESS TEST

Overview — The CST is not used as often as the NST because it is less convenient to perform (see 'Choice of test' above). It is also more limited by contraindications than the NST. Relative contraindications to stimulating contractions for a CST are conditions that are also contraindications to labor and vaginal birth, such as placenta previa, vasa previa, and previous classical cesarean birth or extensive uterine surgery. Preterm labor, patients at high risk for preterm birth, and preterm prelabor rupture of membranes are also relative contraindications.

Procedure — For the CST, either a dilute oxytocin solution is infused or nipple stimulation is performed until three contractions occur within 10 minutes. There is no standard technique for nipple stimulation. Patients gently massage the nipple of one breast through their clothes for two minutes, stopping with onset of contractions; stimulation is resumed if contractions are too infrequent for CST interpretation. Both nipples can be stimulated if no contractions occur. In patients who are having spontaneous contractions of adequate frequency, oxytocin or nipple stimulation is unnecessary.

Interpretation — The CST is interpreted as follows [13]:

Positive – A positive (abnormal) test has late decelerations following ≥50 percent of contractions. The test is positive even if the contraction frequency is less than three in 10 minutes.

Negative – A negative (normal) test has no late decelerations or significant variable decelerations.

Equivocal – An equivocal-suspicious test has intermittent late decelerations or significant variable decelerations, while an equivocal-tachysystolic has decelerations with contractions occurring more frequently than every two minutes or lasting longer than 90 seconds.

Unsatisfactory – An unsatisfactory test has fewer than three contractions in 10 minutes (and is not positive as defined above), or is uninterpretable for other reasons.

The presence or absence of accelerations is also generally noted. For example, a reactive positive CST meets criteria for both a reactive NST and a positive CST.

The American College of Obstetricians and Gynecologists has not defined what they intended by the term "significant" variable decelerations. No data are available on which to base a recommendation for the definition of "significant variable" in the context of antepartum testing. The NICHD chose not to grade the severity of intrapartum variable decelerations based on the depth of the deceleration, absolute nadir, or duration [11]. However, one expert group defined significant variables during labor as those that include either of the following characteristics: lasting >60 seconds and reaching a nadir >60 bpm below baseline or lasting >60 seconds and reaching a nadir <60 bpm (regardless of baseline) [56].

Evaluation of pregnancies with positive CSTs — A positive (abnormal) CST indicates transient fetal hypoxemia during uterine contractions and may be an indication for delivery, depending on the clinical scenario. Further evaluation might include a biophysical profile (BPP) and, in the setting of fetal growth restriction, Doppler velocimetry. A CST with variable decelerations suggests umbilical cord compression that may be due to oligohydramnios [29]. Management of pregnancies complicated by oligohydramnios is reviewed separately. (See "Oligohydramnios: Etiology, diagnosis, and management in singleton gestations".)

In one study, 50 percent of reactive positive CSTs were false positives, whereas 100 percent of nonreactive positive CSTs were true positives [57]. Thus, the presence of accelerations during the CST may reduce the need for intervention, but additional evaluation (eg, BPP) is warranted [57,58].

SOCIETY GUIDELINE LINKS — Links to society and government-sponsored guidelines from selected countries and regions around the world are provided separately. (See "Society guideline links: Fetal surveillance".)

SUMMARY AND RECOMMENDATIONS

Antenatal fetal surveillance

Indications – Antepartum fetal surveillance (antepartum testing) with the nonstress test (NST) or the contraction stress test (CST) is performed in pregnancies considered to be at increased risk for fetal hypoxic injury or demise. (See 'Indications' above.)

Goals – The primary goal of antepartum testing is to reduce the risk of stillbirth by identifying fetuses at risk for hypoxic injury or death so that appropriate intervention (eg, measures directed at improving fetal oxygenation, delivery) might be undertaken to prevent these adverse outcomes, if possible. The secondary goal is to identify normally oxygenated fetuses so that pregnancy can be continued safely, and unnecessary intervention can be avoided. (See 'Introduction' above.)

Physiology – Changes in the fetal heart rate (FHR) reflect the fetal response to input from chemoreceptors, baroreceptors, central nervous system activities, hormonal regulation, and blood volume. Gestational age is a factor in interpretation of FHR patterns. (See 'Physiologic basis of fetal heart rate changes' above.)

Timing and frequency – NSTs and CSTs are initiated at the gestational age that the fetus is believed to be at increased risk of death as long as the age is sufficiently advanced that delivery for perinatal benefit would be considered. The frequency of testing depends upon maternal and fetal status and is usually performed once or twice weekly as long as the indication for testing persists. Maternal or fetal deterioration requires reevaluation despite recent normal test results. (See 'Timing and frequency' above.)

Choice of test – The NST (with or without amniotic fluid assessment) has become the preferred antepartum test of fetal oxygenation status because the CST, which usually requires stimulation of uterine contractions, is more time-consuming, more invasive, and more limited by contraindications than the NST. The biophysical profile (BPP) is a reasonable alternative. (See 'Choice of test' above.)

Abnormal testing – An abnormal test result is generally followed by additional testing with a different test, given the high rate of false-positive results. (See 'Management of pregnancies with normal or abnormal test results' above.)

Nonstress test

≥32 weeks of gestation – The NST is considered reactive if there are two or more FHR accelerations reaching a peak of at least 15 beats per minute (bpm) above the baseline rate and lasting at least 15 seconds from onset to return to baseline in a 20-minute period (figure 2). There are no published data to confirm a benefit of extending the duration of NST once two qualifying accelerations have been observed. A reactive test reliably predicts normal fetal oxygenation at the time of testing. (See 'Reactive tests' above.)

<32 weeks of gestation – Before 32 weeks, a reactive NST can be defined by at least two accelerations of at least 10 bpm lasting for at least 10 seconds and over a 20-minute interval. Once a fetus has demonstrated the maturity to have accelerations of 15 bpm for 15 seconds, an acceleration of 10 bpm for 10 seconds may no longer be sufficient to confirm normal fetal oxygenation, even if less than 32 weeks of gestation. (See 'Reactive tests' above.)

Nonreactive –The FHR should be monitored for at least 40 minutes before interpreting the test as nonreactive. Abnormal test results may be due to interruption of fetal oxygenation but should be interpreted within the context of the clinical situation. Vibroacoustic stimulation is useful for decreasing the number of nonreactive NSTs and for shortening test time. (See 'Nonreactive tests' above and 'Evaluation of pregnancies with nonreactive NSTs' above.)

Reactive with decelerations – Variable, late, or prolonged decelerations observed during a NST require further evaluation (table 1). (See 'Reactive NSTs with decelerations' above.)

Contraction stress test – The CST is interpreted as follows (see 'Contraction stress test' above):

Positive – Late decelerations following ≥50 percent of contractions, even if the contraction frequency is less than three in 10 minutes.

Negative – No late decelerations or significant variable decelerations with contraction frequency of three in 10 minutes.

Equivocal – Intermittent late decelerations or significant variable decelerations, while an equivocal-tachysystolic has decelerations with contractions occurring more frequently than every two minutes or lasting longer than 90 seconds.

Unsatisfactory – Uninterpretable or fewer than three contractions in 10 minutes.

Reactive positive CSTs are often false positives, whereas nonreactive positive CSTs are generally true positives. Thus, the presence of accelerations during the CST may reduce the need for intervention, but additional evaluation (eg, BPP) is warranted. (See 'Evaluation of pregnancies with positive CSTs' above.)

ACKNOWLEDGMENT — The UpToDate editorial staff acknowledges Bruce K Young, MD, who contributed to earlier versions of this topic review.

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