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Contraction stress test
Contraction stress test
Contraction stress test
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Contraction stress test
Purposefetal test (heart rate)

A contraction stress test (CST) is performed near the end of pregnancy (34 weeks' gestation) to determine how well the fetus will cope with the contractions of childbirth. The aim is to induce contractions and monitor the fetus to check for heart rate abnormalities using a cardiotocograph. A CST is one type of antenatal fetal surveillance technique.

During uterine contractions, fetal oxygenation is worsened. Late decelerations in fetal heart rate occurring during uterine contractions are associated with increased fetal death rate, growth retardation and neonatal depression.[1][2] This test assesses fetal heart rate in response to uterine contractions via electronic fetal monitoring. Uterine activity is monitored by tocodynamometer.[3]

Medical uses

[edit]

The CST is used for its high negative predictive value. A negative result is highly predictive of fetal wellbeing and tolerance of labor. The test has a poor positive predictive value with false-positive results in as many as 30% of cases.[4][5] A positive CST indicates high risk of fetal death due to hypoxia[3] and is a contraindication to labor. Patient's obstetricians usually consider operative delivery in such situations.[6]

Contraindications

[edit]

This "stress test" is usually not performed if there are any signs of premature birth, placenta praevia, vasa praevia, cervical incompetence, multiple gestation, previous classic caesarian section.[1] Other contraindications include but are not limited to previous uterine incision with scarring, previous myomectomy entering the uterine cavity, and PROM. Any contraindication to labor is contraindication to CST.

Procedure

[edit]

CST is performed weekly,[1] as the fetus is assumed to be healthy after a negative test and should remain so for another week.[7] This test is done in hospital or clinic setting.[3] External fetal monitors are put in place and then either nipple stimulation or IV pitocin (oxytocin) is used to stimulate uterine contractions.

Nipple stimulation

[edit]

This is a procedure that relies on endogenous release of oxytocin following nipple stimulation, and is conducted by the patient. The nurse instructs the patient on the procedure, as follows. One nipple is massaged gently through clothing until a contraction begins, or for a maximum of 2 minutes. If at least 3 contractions in 10 minutes is not achieved, then the patient rests for 5 minutes and the other nipple is stimulated.[8][4]

Oxytocin challenge test (OCT)

[edit]

If adequate contractions (at least 3 in 10 minutes) cannot be achieved with nipple stimulation, an oxytocin challenge test may be performed. It involves the intravenous administration of exogenous oxytocin to the pregnant woman. The target is to achieve around three contractions every ten minutes.[8][4]

Interpretation

[edit]
Result Interpretation
Positive Presence of late decelerations with at least 50% of the contractions[9]
Negative No late or significant variable decelerations,[9] with at least 3 uterine contractions (lasting 40 seconds) in 10 minute period.[10]
Equivocal—Suspicious Presence of late decelerations with fewer than 50% of contractions or significant variable decelerations. Requires repeat testing on following day.[1]
Equivocal—Tachysystole Presence of contractions that occur more frequently than every 2 minutes or last longer than 90 seconds in the presence of late decelerations. Requires repeat testing on following day.[1]
Equivocal—Unsatisfactory Fewer than three contractions occur within 10 minutes, or a tracing quality that cannot be interpreted. Requires repeat testing on following day.[1]

History

[edit]

The CST was the first antenatal surveillance test that was developed after the development of the cardiotocograph.[4] The oxytocin challenge test was first described in 1972[11] and was standardised in 1975 when the parameters of contraction number and frequency were given.

Historically, a CST was done after a non reactive NST. Today, a biophysical profile (BPP) is usually performed.

See also

[edit]

References

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[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Contraction stress test

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from Grokipedia
The contraction stress test (CST), also known as the oxytocin challenge test, is a prenatal diagnostic procedure used to assess fetal by monitoring the fetal heart rate (FHR) in response to , simulating the stress of labor to evaluate the fetus's ability to tolerate reduced oxygen and blood flow. This test is typically indicated in high-risk pregnancies, such as those involving post-term , maternal conditions like or , , or following an abnormal (NST), to help determine if early delivery is necessary. It is performed in a clinical setting, usually after 32 weeks of , and is considered a more definitive assessment than the NST when fetal compromise is suspected. A normal CST result indicates that the is likely to withstand labor, while an abnormal result may prompt further evaluation or intervention, such as cesarean delivery. The procedure involves placing external monitors on the mother's abdomen to simultaneously record uterine contractions and FHR patterns. If spontaneous contractions do not occur (defined as at least three contractions lasting 40 seconds or more in a 10-minute window), they are induced either by intravenous infusion of oxytocin (Pitocin) in gradually increasing doses or by nipple stimulation to promote endogenous oxytocin release, mimicking early labor. The test is interpreted based on FHR changes: a negative result shows no late decelerations (a reassuring sign of adequate placental reserve), a positive result features late decelerations after 50% or more of contractions (indicating potential fetal hypoxia), suspicious patterns include intermittent decelerations, and unsatisfactory tracings require repetition due to poor data quality. Although generally safe, the CST carries minor risks, including overstimulation leading to excessive contractions, preterm labor, or fetal distress, and it is contraindicated in conditions like preterm labor, placenta previa, or premature rupture of membranes where poses hazards. Advances in antenatal surveillance have somewhat reduced its routine use in favor of less invasive tests like the , but it remains a valuable tool for equivocal cases.

Overview

Definition and Mechanism

The contraction stress test (CST), also known as the oxytocin challenge test, is a prenatal diagnostic procedure that evaluates the fetal heart rate (FHR) response to induced , thereby assessing placental reserve and the 's ability to tolerate the stress of labor. This test is particularly useful in identifying uteroplacental insufficiency, where the may not adequately supply oxygen and nutrients to the under stress conditions. The underlying mechanism of the CST relies on the physiological effects of , which temporarily reduce blood flow through the intervillous space of the , mimicking the hypoxic stress experienced during labor. In a healthy , the compensates for this brief reduction in oxygenation through cardiovascular autoregulation, maintaining a stable FHR without significant decelerations. However, if exists, the persistent hypoxia during contractions can trigger late FHR decelerations, characterized by a gradual onset and return to baseline after the contraction peak, due to fetal vagal activation and myocardial depression. Fetal monitoring during the CST involves external electronic fetal-maternal monitors, with a Doppler transducer placed on the maternal to continuously record the FHR and a tocodynamometer to detect via changes in intra-abdominal pressure. In cases where external monitoring is inadequate, an internal fetal scalp electrode may be used for more precise FHR tracing. The test aims to achieve a standard pattern of three moderate to strong contractions, each lasting 40 to 60 seconds, within a 10-minute window, sustained for at least 10 minutes to ensure adequate assessment.

Historical Development

The roots of the contraction stress test (CST) trace back to the and , when researchers observed fetal heart rate (FHR) responses to hypoxia and during labor. Pioneering work by Roberto Caldeyro-Barcia and colleagues in identified patterns of FHR decelerations associated with fetal distress, establishing a foundational understanding of how contractions could stress the fetus and reveal uteroplacental insufficiency. The research group, including Caldeyro-Barcia and collaborators, extended these observations, linking late decelerations to reduced fetal oxygenation during contractions, which informed early concepts of for antepartum evaluation. The formal introduction of the CST occurred in 1972, when Roger K. Freeman and colleagues described the oxytocin challenge test (OCT)—the precursor to the modern CST—as a method to assess antepartum fetal reserve by inducing contractions with intravenous oxytocin and monitoring FHR patterns. This approach built on intrapartum monitoring techniques to predict fetal tolerance to labor, with initial studies showing that the absence of late decelerations correlated with favorable outcomes and their presence indicated high risk for or neonatal compromise. By 1975, Freeman's group further refined the protocol, standardizing contraction frequency to three contractions in 10 minutes to ensure adequate stress without excessive risk, which facilitated its widespread clinical adoption as a reliable tool for high-risk pregnancies. In the post-1980s era, the CST transitioned from a primary screening method to an adjunctive role, largely due to the rise of the (NST), introduced in the late 1970s as a simpler, non-invasive alternative relying on spontaneous fetal movements. American College of Obstetricians and Gynecologists (ACOG) guidelines from the 1980s onward emphasized CST's utility in high-risk cases, particularly following nonreactive NSTs, while promoting NST as the initial test to reduce maternal discomfort and logistical burdens. This shift reflected broader advancements in fetal monitoring, including biophysical profiles, leading to decreased routine CST use. As of the 2021 ACOG Practice Bulletin, the CST remains relevant for confirming fetal well-being in select scenarios, such as nonreactive NSTs in high-risk pregnancies, though its frequency has diminished with evidence favoring alternatives like the for comprehensive assessment.

Clinical Applications

Indications

The contraction stress test (CST) is primarily indicated in high-risk pregnancies where there is an elevated risk of fetal compromise, such as post-term beyond 41 weeks, maternal (pregestational or gestational), hypertensive disorders including chronic or , (IUGR), , and a history of prior . These conditions are associated with increased , and CST helps assess fetal tolerance to the stress of uterine contractions to guide timely intervention. CST is also used adjunctively as a follow-up test when initial screening methods yield equivocal or non-reassuring results, such as a nonreactive (NST) or an abnormal (BPP), to further evaluate fetal well-being and placental reserve. In these scenarios, it provides additional diagnostic clarity without immediately resorting to delivery. The test is typically performed after 32-34 weeks of , when the risks of preterm delivery are balanced against the benefits of intervention if abnormalities are detected. Frequency is tailored to the underlying risk factors; for example, weekly is recommended in uncomplicated post-term pregnancies, while more frequent testing (e.g., twice weekly) may be warranted in cases of worsening maternal conditions like uncontrolled or . According to the American College of Obstetricians and Gynecologists (ACOG) Practice Bulletin No. 229 (2021) and Committee Opinion No. 828 (2021), CST plays a key role in antepartum fetal surveillance for equivocal cases, particularly when other tests are inconclusive, as part of a broader strategy to minimize risk in high-risk populations.

Contraindications

The contraction stress test (CST) has specific absolute contraindications, which are conditions where performing the test is deemed unsafe due to the high risk of serious maternal or fetal harm from induced uterine contractions. These include placenta previa, where contractions could precipitate severe hemorrhage from the low-lying placenta; vasa previa, in which fetal blood vessels traverse the cervical os and are vulnerable to rupture; a history of classical uterine scar from prior cesarean section, increasing the risk of uterine rupture; active genital herpes infection, due to the potential for viral transmission during membrane manipulation or contractions; and preterm premature rupture of membranes, due to infection risk. Relative contraindications for CST encompass scenarios where the test may be considered with caution if the potential benefits outweigh the risks, but alternatives are often preferred to avoid exacerbating underlying issues. These include a history of preterm labor or current risk of preterm labor before 34 weeks' gestation, as induced contractions may trigger premature delivery; multiple gestation with significant complications, such as those involving preterm risks or abnormal presentations that could lead to unpredictable labor induction; a prior category III fetal heart rate (FHR) tracing indicating severe fetal compromise; and maternal conditions such as severe asthma or cardiovascular disease, where oxytocin administration could provoke bronchospasm, arrhythmias, or hemodynamic instability. The primary rationale for these contraindications stems from the mechanism of CST, which simulates labor by inducing contractions and thereby stresses fetal oxygenation; in contraindicated conditions, this can lead to catastrophic outcomes such as and hemorrhage in previa cases, vascular rupture in vasa previa, uterine dehiscence in scarred uteri, acceleration, or maternal decompensation in cardiopulmonary disorders. According to guidelines from the American College of Obstetricians and Gynecologists (ACOG), CST should be deferred in the presence of these contraindications, with nonstress testing (NST) or other methods recommended as safer alternatives to assess fetal without the risks of contraction induction.

Procedure

Nipple Stimulation Method

The nipple stimulation method serves as a non-pharmacological approach to induce during a contraction stress test (CST), releasing endogenous oxytocin to mimic natural labor patterns without the need for intravenous administration. This technique is particularly valued for its simplicity and reduced invasiveness compared to the oxytocin challenge method. Patient preparation involves ensuring an empty to optimize uterine monitoring and positioning the in a semi-recumbent or lateral position for comfort and effective fetal heart rate (FHR) tracing. is obtained, explaining the procedure's sensations, which may include tingling or mild discomfort, and the rare possibility of initiating true labor. The protocol typically begins with a baseline FHR and uterine activity tracing using external electronic fetal monitoring. The woman is instructed to gently roll or one between her thumb and forefinger, or use a , for 2 to 3 minutes per , alternating sides if necessary. This cycle is followed by a 5-minute rest period, repeated up to a total duration of 20 to 40 minutes or until the desired contraction pattern is achieved. can be performed through clothing to maintain . Continuous external monitoring of FHR and uterine activity occurs throughout the procedure via Doppler ultrasound and tocodynamometer. The test proceeds until three contractions of moderate intensity, lasting at least 40 seconds each, occur within a 10-minute window; it is halted if hyperstimulation (more than five contractions in 10 minutes) develops or if fetal distress is noted. Advantages include its feasibility in outpatient settings, elimination of intravenous access, and lower associated costs compared to pharmacological alternatives. Success rates in achieving adequate contractions are high, reported at approximately 93% in term pregnancies in early studies.

Oxytocin Challenge Method

The oxytocin challenge method, also known as the oxytocin challenge test (OCT), involves the intravenous administration of oxytocin to pharmacologically induce uterine contractions for assessing fetal heart rate (FHR) response in high-risk pregnancies, particularly when spontaneous contractions are insufficient or nipple stimulation is not feasible or successful. This approach is employed in a controlled hospital environment to simulate labor stress and evaluate uteroplacental function. Patient preparation for the procedure includes establishing intravenous (IV) access, ensuring the patient is nil per os (NPO) to minimize aspiration risk, and confirming readiness for cesarean delivery, including availability of and operating room resources. The patient is positioned in the lateral recumbent position to avoid aortocaval compression and supine hypotension. Continuous electronic fetal monitoring is initiated with transducers for FHR and tocodynamometry to record uterine activity prior to oxytocin infusion. The protocol begins with a 20-minute baseline (NST) to assess FHR reactivity. If fewer than three contractions lasting at least 40 seconds occur in 10 minutes, diluted oxytocin (typically 10-20 units in 1,000 mL of IV fluid) is infused starting at 0.5-1 mU/min. The dose is increased incrementally by 1-2 mU/min every 15-30 minutes until the target of three contractions in 10 minutes is achieved, with a maximum dose of 20-40 mU/min to avoid excessive stimulation. Once adequate contractions are attained, monitoring continues for at least 30-40 minutes to evaluate FHR patterns. Throughout the procedure, FHR and contraction patterns are monitored continuously in a hospital setting equipped for immediate intervention. The is discontinued immediately if (contractions more frequent than every 2 minutes or lasting over 90 seconds) occurs or if abnormal FHR patterns, such as late decelerations or prolonged , are observed. Post-adequate tracing, the oxytocin dose is reduced to maintain the contraction pattern or stopped if the test is complete. This method offers higher reliability in achieving an adequate contraction pattern, with success rates approaching 90%, compared to non-pharmacological alternatives, and has been utilized for research standardization since 1975, when criteria for contraction frequency were formalized to enhance reproducibility. It is particularly valuable for its ability to provoke contractions in cases where fails, providing a more consistent assessment of fetal reserve.

Results and Management

Interpretation Criteria

The interpretation of a contraction stress test (CST) relies on analyzing fetal heart rate (FHR) patterns in relation to induced , with a focus on the presence and characteristics of late decelerations. Late decelerations are defined as a gradual, symmetric decrease in FHR of at least 15 beats per minute (bpm) below the baseline, with the onset to lasting 30 seconds or more, beginning at or after the peak of the contraction, and returning to baseline after the contraction ends. These patterns reflect potential fetal responses to uterine and placental , and the test requires at least three contractions of moderate intensity in a 10-minute window for adequate evaluation. A negative CST result occurs when there are no late decelerations (and typically no significant variable decelerations) accompanying at least three contractions in 10 minutes, indicating preserved fetal reserve and a low risk of intrauterine fetal demise within one week, with a false-negative rate of approximately 0.04%. In contrast, a positive result is characterized by late decelerations occurring with 50% or more of the contractions, suggesting possible uteroplacental insufficiency and compromised fetal oxygenation. Suspicious or equivocal results include late decelerations with fewer than 50% of contractions, persistent variable decelerations, or hyperstimulation (more than five contractions in 10 minutes), which may indicate intermittent fetal compromise and necessitate repeat testing or further assessment. An unsatisfactory tracing arises from fewer than three contractions in 10 minutes despite stimulation efforts, or from poor FHR tracing quality that obscures interpretation, requiring reattempting the test or alternative methods.

Clinical Implications

A negative contraction stress test (CST) result indicates reassuring fetal well-being, with a low risk of stillbirth, reported as less than 1 per 1000 tested cases for preventable perinatal death. Management typically involves continuing routine antenatal surveillance, such as weekly nonstress tests (NST), without altering the frequency of monitoring unless other clinical factors arise. A positive CST result, characterized by late decelerations following at least 50% of contractions, suggests potential fetal compromise and warrants immediate intervention. If the fetus is viable at greater than 34 weeks' gestation, delivery is recommended via induction of labor or cesarean section, depending on maternal and fetal status; for preterm gestations, antenatal corticosteroids are administered to enhance fetal lung maturity prior to delivery. A suspicious CST result, involving intermittent or inconsistent late decelerations, serves as a marker for heightened and requires further assessment rather than immediate action. Management includes repeating the CST within 24 hours or performing adjunct tests such as the (BPP), alongside close inpatient monitoring to clarify fetal status. The prognostic accuracy of the CST highlights its high negative predictive value for adverse perinatal outcomes, while the positive predictive value ranges from 13% to 45% for specific complications such as neonatal morbidity or NICU admission, though it can reach up to 28% for in positive cases. In high-risk pregnancies, CST results influence approximately 20-30% of delivery decisions by guiding timing and mode, particularly in cases of suspected uteroplacental insufficiency. Follow-up after CST integrates with broader antepartum testing protocols per American College of Obstetricians and Gynecologists (ACOG) guidelines, emphasizing individualized timing based on and risk factors, with no major updates beyond the 2021 Practice Bulletin as of 2025.

Risks and Alternatives

Potential Risks

The contraction stress test (CST) carries a low overall risk profile, but potential complications primarily stem from induced . , characterized by excessive contractions exceeding five in 10 minutes or lasting longer than 90 seconds, can compromise fetal oxygenation and lead to hypoxia. This occurs in approximately 2.8% of cases involving oxytocin administration for the test. In nipple stimulation variants, rates of tachysystole (contractions more frequent than every two minutes) reach up to 45%, with hyperstimulation accompanied by fetal decelerations reported in 21% of instances. Management involves immediate cessation of stimulation, with contractions typically resolving without intervention; no significant adverse fetal outcomes were observed in affected cases. A key concern is the potential induction of preterm labor, particularly in gestations under 37 weeks, where the test may trigger true labor onset. Incidence rates are low, around 1-2%, though one study of 389 patients undergoing testing before 38 weeks reported spontaneous preterm delivery within five days in 6.7%, comparable to baseline rates of 7.5-7.6% without increased risk attributable to the test. Other complications include maternal discomfort from contractions, which is common but transient, and rare events such as from fetal monitoring equipment or , contributing to an overall morbidity rate below 1%. Risks are mitigated through adherence to strict protocols, including continuous fetal and uterine activity monitoring, incremental dosing of oxytocin if used, and avoidance in contraindicated cases such as prior or multiple gestation. In uncomplicated scenarios, no long-term fetal effects have been documented. Evidence from clinical studies indicates that CST use in high-risk pregnancies reduces to low levels (e.g., approximately 3 per 1,000), compared to unmonitored cohorts, though positive results may slightly elevate intervention rates, including cesarean deliveries in up to 40% of cases.

Comparison to Other Tests

The contraction stress test (CST) offers a more definitive evaluation of placental function and fetal response to hypoxic stress compared to the (NST), as it induces to observe fetal (FHR) decelerations, but it is more invasive and carries risks such as preterm . The NST, monitoring FHR reactivity to fetal movements without contractions, serves as the first-line antepartum surveillance method due to its simplicity and high negative predictive value of approximately 99% for within one week after a reactive result. However, CST demonstrates higher false-positive rates than NST, potentially resulting in unnecessary interventions like delivery. In contrast to the (BPP), which uses to assess fetal movements, body movements, tone, and volume often in combination with NST components for a holistic non-invasive , the CST targets FHR changes specifically during contractions to gauge uteroplacental reserve. The modified BPP is generally preferred over CST for follow-up after a nonreactive NST, providing broader insights into fetal without the need for contraction stimulation. Whereas Doppler velocimetry directly measures blood flow velocity and resistance to detect placental vascular issues, the CST assesses the fetus's overall functional reserve under stress, and the two methods are complementary in managing (IUGR) by combining hemodynamic and stress-response data. Per ACOG Practice Bulletin No. 229 (2021), the CST is recommended primarily as a confirmatory test after a nonreactive NST in high-risk pregnancies, though its utilization has declined due to the greater safety, ease, and availability of alternatives like NST and BPP. The CST's advantage lies in its superior sensitivity for identifying fetal hypoxia through simulated labor stress, outperforming NST in specificity for , but it is more resource-intensive, often lasting 30 to 90 minutes versus 20 to 40 minutes for NST.

References

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  14. https://www.acog.org/clinical/clinical-guidance/committee-opinion/articles/2021/06/indications-for-outpatient-antenatal-fetal-surveillance
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  18. https://www.uwmedicine.org/sites/stevie/files/2020-08/Antenatal%20Outpatient%20Testing%20-%202020_2.pdf
  19. https://pubmed.ncbi.nlm.nih.gov/6717870/
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