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Contraction stress test

The contraction stress test (CST), also known as the oxytocin challenge test, is a prenatal diagnostic procedure used to assess fetal well-being by monitoring the fetal heart rate (FHR) in response to uterine contractions, simulating the stress of labor to evaluate the fetus's ability to tolerate reduced oxygen and blood flow.^[1]^[2]^[3] This test is typically indicated in high-risk pregnancies, such as those involving post-term gestation, maternal conditions like hypertension or diabetes, intrauterine growth restriction, or following an abnormal nonstress test (NST), to help determine if early delivery is necessary.^[1]^[2] It is performed in a clinical setting, usually after 32 weeks of gestation, and is considered a more definitive assessment than the NST when fetal compromise is suspected.^[2] A normal CST result indicates that the fetus is likely to withstand labor, while an abnormal result may prompt further evaluation or intervention, such as cesarean delivery.^[3] 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.^[1]^[2]^[3] 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.^[2] 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 vaginal delivery poses hazards.^[2] Advances in antenatal surveillance have somewhat reduced its routine use in favor of less invasive tests like the biophysical profile, but it remains a valuable tool for equivocal cases.^[2]

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 uterine contractions, thereby assessing placental reserve and the fetus's ability to tolerate the stress of labor.^[4] This test is particularly useful in identifying uteroplacental insufficiency, where the placenta may not adequately supply oxygen and nutrients to the fetus under stress conditions.^[5] The underlying mechanism of the CST relies on the physiological effects of uterine contractions, which temporarily reduce blood flow through the intervillous space of the placenta, mimicking the hypoxic stress experienced during labor. In a healthy pregnancy, the fetus compensates for this brief reduction in oxygenation through cardiovascular autoregulation, maintaining a stable FHR without significant decelerations. However, if placental insufficiency 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 reflex activation and myocardial depression.^[4]^[6] Fetal monitoring during the CST involves external electronic fetal-maternal monitors, with a Doppler ultrasound transducer placed on the maternal abdomen to continuously record the FHR and a tocodynamometer to detect uterine contractions 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.^[4]^[5]^[6]

Historical Development

The roots of the contraction stress test (CST) trace back to the 1950s and 1960s, when researchers observed fetal heart rate (FHR) responses to hypoxia and uterine contractions during labor. Pioneering work by Roberto Caldeyro-Barcia and colleagues in Uruguay identified patterns of FHR decelerations associated with fetal distress, establishing a foundational understanding of how contractions could stress the fetus and reveal uteroplacental insufficiency.^[7] The Montevideo research group, including Caldeyro-Barcia and collaborators, extended these observations, linking late decelerations to reduced fetal oxygenation during contractions, which informed early concepts of stress testing for antepartum evaluation.^[8] 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.^[9] 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 stillbirth 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.^[10] In the post-1980s era, the CST transitioned from a primary screening method to an adjunctive role, largely due to the rise of the nonstress test (NST), introduced in the late 1970s as a simpler, non-invasive alternative relying on spontaneous fetal movements.^[11] 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 biophysical profile for comprehensive assessment.^[12]

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 gestation beyond 41 weeks, maternal diabetes (pregestational or gestational), hypertensive disorders including chronic hypertension or preeclampsia, intrauterine growth restriction (IUGR), oligohydramnios, and a history of prior stillbirth.^[13] These conditions are associated with increased perinatal mortality, and CST helps assess fetal tolerance to the stress of uterine contractions to guide timely intervention.^[13] CST is also used adjunctively as a follow-up test when initial screening methods yield equivocal or non-reassuring results, such as a nonreactive nonstress test (NST) or an abnormal biophysical profile (BPP), to further evaluate fetal well-being and placental reserve.^[14] In these scenarios, it provides additional diagnostic clarity without immediately resorting to delivery.^[14] The test is typically performed after 32-34 weeks of gestation, when the risks of preterm delivery are balanced against the benefits of intervention if abnormalities are detected.^[13] Frequency is tailored to the underlying risk factors; for example, weekly surveillance 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 diabetes or hypertension.^[14] 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 stillbirth risk in high-risk populations.^[13]^[14]

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.^[4]^[15]^[2]^[16]^[6] 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.^[4]^[15]^[17]^[16] 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 placental abruption and hemorrhage in previa cases, vascular rupture in vasa previa, uterine dehiscence in scarred uteri, preterm birth acceleration, or maternal decompensation in cardiopulmonary disorders.^[4]^[2]^[15] 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 surveillance methods recommended as safer alternatives to assess fetal well-being without the risks of contraction induction.^[2]^[18]

Procedure

Nipple Stimulation Method

The nipple stimulation method serves as a non-pharmacological approach to induce uterine contractions during a contraction stress test (CST), releasing endogenous oxytocin to mimic natural labor patterns without the need for intravenous administration.^[1] This technique is particularly valued for its simplicity and reduced invasiveness compared to the oxytocin challenge method.^[4] Patient preparation involves ensuring an empty bladder to optimize uterine monitoring and positioning the woman in a semi-recumbent or lateral position for comfort and effective fetal heart rate (FHR) tracing.^[1] Informed consent is obtained, explaining the procedure's sensations, which may include tingling or mild discomfort, and the rare possibility of initiating true labor.^[4] 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 massage one nipple between her thumb and forefinger, or use a breast pump, for 2 to 3 minutes per breast, alternating sides if necessary.^[19] This stimulation 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.^[1] Stimulation can be performed through clothing to maintain modesty.^[19] 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.^[4] 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.^[19]

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.^[12] This approach is employed in a controlled hospital environment to simulate labor stress and evaluate uteroplacental function.^[2] 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 emergency cesarean delivery, including availability of anesthesia and operating room resources.^[15] The patient is positioned in the lateral recumbent position to avoid aortocaval compression and supine hypotension.^[2] Continuous electronic fetal monitoring is initiated with transducers for FHR and tocodynamometry to record uterine activity prior to oxytocin infusion.^[12] The protocol begins with a 20-minute baseline nonstress test (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.^[15] 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.^[20] Once adequate contractions are attained, monitoring continues for at least 30-40 minutes to evaluate FHR patterns.^[15] Throughout the procedure, FHR and contraction patterns are monitored continuously in a hospital setting equipped for immediate intervention. The infusion is discontinued immediately if uterine hyperstimulation (contractions more frequent than every 2 minutes or lasting over 90 seconds) occurs or if abnormal FHR patterns, such as late decelerations or prolonged bradycardia, are observed.^[12] Post-adequate tracing, the oxytocin dose is reduced to maintain the contraction pattern or stopped if the test is complete.^[15] 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.^[21] It is particularly valuable for its ability to provoke contractions in cases where nipple stimulation fails, providing a more consistent assessment of fetal reserve.^[20]

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 uterine contractions, 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 nadir lasting 30 seconds or more, beginning at or after the peak of the contraction, and returning to baseline after the contraction ends.^[22]^[23] These patterns reflect potential fetal responses to uterine and placental physiology, and the test requires at least three contractions of moderate intensity in a 10-minute window for adequate evaluation.^[1]^[2] 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%.^[1]^[24] 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.^[2]^[1] 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.^[2] 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.^[1]^[2]

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.^[25] Management typically involves continuing routine antenatal surveillance, such as weekly nonstress tests (NST), without altering the frequency of monitoring unless other clinical factors arise.^[12] 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.^[12]^[2] A suspicious CST result, involving intermittent or inconsistent late decelerations, serves as a marker for heightened risk and requires further assessment rather than immediate action. Management includes repeating the CST within 24 hours or performing adjunct tests such as the biophysical profile (BPP), alongside close inpatient monitoring to clarify fetal status.^[26]^[27] 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 perinatal mortality in positive cases.^[28]^[29] 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.^[2] Follow-up after CST integrates with broader antepartum testing protocols per American College of Obstetricians and Gynecologists (ACOG) guidelines, emphasizing individualized timing based on gestational age and risk factors, with no major updates beyond the 2021 Practice Bulletin as of 2025.^[12]

Risks and Alternatives

Potential Risks

The contraction stress test (CST) carries a low overall risk profile, but potential complications primarily stem from induced uterine contractions. Uterine hyperstimulation, 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.^[30] In nipple stimulation variants, rates of tachysystole (contractions more frequent than every two minutes) reach up to 45%, with hyperstimulation accompanied by fetal heart rate decelerations reported in 21% of instances.^[31] 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.^[32] Other complications include maternal discomfort from contractions, which is common but transient, and rare events such as infection from fetal monitoring equipment or cord prolapse, contributing to an overall morbidity rate below 1%. Risks are mitigated through adherence to strict protocols, including continuous fetal heart rate and uterine activity monitoring, incremental dosing of oxytocin if used, and avoidance in contraindicated cases such as prior uterine rupture 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 perinatal mortality 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.^[33]

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 nonstress test (NST), as it induces uterine contractions to observe fetal heart rate (FHR) decelerations, but it is more invasive and carries risks such as preterm labor induction.^[34] 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 stillbirth within one week after a reactive result.^[34] However, CST demonstrates higher false-positive rates than NST, potentially resulting in unnecessary interventions like delivery.^[2] In contrast to the biophysical profile (BPP), which uses ultrasound to assess fetal breathing movements, body movements, tone, and amniotic fluid volume often in combination with NST components for a holistic non-invasive evaluation, the CST targets FHR changes specifically during contractions to gauge uteroplacental reserve.^[12] The modified BPP is generally preferred over CST for follow-up after a nonreactive NST, providing broader insights into fetal well-being without the need for contraction stimulation.^[13] Whereas Doppler velocimetry directly measures umbilical artery 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 intrauterine growth restriction (IUGR) by combining hemodynamic and stress-response data.^[2] 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.^[12] The CST's advantage lies in its superior sensitivity for identifying fetal hypoxia through simulated labor stress, outperforming NST in specificity for placental insufficiency, but it is more resource-intensive, often lasting 30 to 90 minutes versus 20 to 40 minutes for NST.^[34]

References

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