Contraction Stress Test: Purpose, Test Details & Results

The contraction stress test (CST) is an important tool in antenatal care, especially for pregnancies with potential complications. While newer fetal monitoring tests have emerged, CST remains valuable in specific clinical situations. This article provides a comprehensive overview of the contraction stress test, including its purpose, methodology, and interpretation of results. Whether you are a patient seeking to understand your care options or a clinician looking for an up-to-date synthesis, this guide covers the essentials and nuances of CST, referencing peer-reviewed sources throughout.

Purpose of Contraction Stress Test

Understanding the reasons behind the contraction stress test is critical for both expectant parents and healthcare providers. The CST is primarily designed to assess fetal well-being by evaluating how the baby's heart rate responds to the reduced oxygen supply that can occur during uterine contractions. This information helps clinicians make informed decisions about the timing and mode of delivery, especially in high-risk pregnancies.

Assessing Fetal Reserve and Tolerance to Labor

The contraction stress test is particularly useful for evaluating the "fetal reserve"—the fetus's ability to tolerate the temporary decrease in oxygen that occurs during uterine contractions. By observing the fetal heart rate response to induced contractions, clinicians can predict how the fetus might cope with the stresses of labor. A negative CST generally indicates that the fetus is likely to handle labor well if it occurs within the following week, providing reassurance for both the medical team and parents 1.

Primary Populations: High-Risk and Postdate Pregnancies

CST is most beneficial in pregnancies that are considered high risk. This includes women with hypertensive disorders, those who have reached or surpassed 42 weeks of gestation (postdate pregnancies), and cases where fetal growth restriction (FGR) is suspected 1 2 11. In such scenarios, the test can help identify babies who may be at increased risk of distress during labor, allowing for early intervention if needed.

Reducing Perinatal Morbidity and Mortality

By identifying fetuses at risk of compromised oxygenation before labor begins, the CST serves as a primary surveillance tool. Its aim is to prevent adverse outcomes—such as intrapartum fetal distress, low Apgar scores, or the need for emergency cesarean delivery—by guiding timely and appropriate obstetric management 2 11.

Contraction Stress Test Details

The CST is a structured test, usually performed in a hospital setting, that simulates the stress of labor contractions to monitor fetal well-being. Understanding how the test is performed and the different techniques used can help demystify the process and prepare patients for what to expect.

How the Test Is Performed

The CST involves inducing uterine contractions in women who are not in labor, then monitoring the fetus’s heart rate in response to these contractions. The process typically includes:

• Placement of external monitors: One to track fetal heart rate, and another (tocograph) to measure uterine contractions 1.
• Induction of contractions: This can be achieved in two main ways:
  • Oxytocin infusion: Intravenous administration of oxytocin to stimulate contractions. This is the traditional method, but it requires IV access, is more resource-intensive, and can be uncomfortable for the patient 1 4 5.
  • Nipple or breast stimulation: The patient stimulates one or both nipples (often through clothing) in cycles until adequate contractions are achieved. This method is non-invasive, faster, and less expensive, with a high success rate of producing contractions sufficient for the test 4 5 8 10.
• The goal is to achieve three contractions within a 10-minute window, each lasting at least 40 seconds, to adequately assess the fetal response 5 8.

Advantages of Nipple Stimulation

Recent studies support the use of nipple or breast stimulation as an effective alternative to oxytocin-induced CSTs. This method is associated with:

• High rates of successful testing (over 90%)
• Reduced time to complete the test (mean duration as low as 12 minutes)
• Lower cost and increased patient comfort
• Reduced need for IV interventions and associated risks 4 5 8 10

Safety and Patient Experience

CST is generally safe when performed in a controlled environment. Hyperstimulation (i.e., too many contractions) can occur but is carefully monitored, and the test is stopped if the fetus shows signs of distress. The non-invasive approaches further minimize risks and discomfort 4 8 12.

Who Should (and Shouldn’t) Have a CST

CST is indicated for women with high-risk features (see Table 1) but is not recommended for those with contraindications to labor, such as placenta previa, previous classical cesarean section, or other reasons where inducing contractions would be unsafe.

Contraction Stress Test Results & Follow-Up

The true value of the CST lies in its interpretation and the clinical decisions that follow. Understanding what test results mean and how they impact ongoing care is crucial for optimizing outcomes.

Interpreting CST Results

• Negative CST: No late decelerations of the fetal heart rate with contractions. This is a reassuring result, indicating the fetus is likely to tolerate labor without significant distress if delivery occurs within about one week 1 2 8 10.
• Positive CST: Late decelerations occur with more than half of the contractions, suggesting the fetus may not handle the stress of labor well and may benefit from early delivery or closer monitoring 1 2 11.
• Suspicious (or "Equivocal") CST: Variable, inconsistent, or non-specific changes in fetal heart rate. This may require repeating the test or additional assessments, as these results can indicate evolving fetal compromise or may be technically limited 9 12.
• Equivocal/Hyperstimulation CST: Excessive contractions or technical limitations make the result unclear. In such cases, repeating the test within 24 hours is often recommended 12.

What Happens After the Test?

The test results directly influence clinical management:

• Negative CST: Routine surveillance continues, with the test often repeated weekly if the pregnancy remains at risk 1 2 8 10.
• Positive CST: May prompt delivery or more intensive fetal monitoring, especially in post-term or growth-restricted pregnancies 1 2 11.
• Suspicious/Equivocal CST: Additional testing (such as nonstress tests or biophysical profiles), or repeating the CST, is usually recommended 9 12.

Predictive Value and Outcome

Studies have shown that a negative CST is a strong predictor of good perinatal outcome in the week following the test, with very low perinatal mortality rates 1 2 10. Positive CST results are associated with higher rates of cesarean delivery due to fetal distress, especially in post-term pregnancies and cases of fetal growth restriction 2 11 12.

Notably, in high-risk populations such as those with fetal growth restriction, CST may help guide timely intervention before the fetus develops more severe complications such as acidosis or low oxygen levels 11. However, the test should always be interpreted in the context of the overall clinical picture and in conjunction with other fetal surveillance tools.

Conclusion

The contraction stress test remains a valuable, evidence-based tool in the management of high-risk pregnancies. It provides critical information about fetal well-being and helps guide decisions about timing and mode of delivery. Here’s a recap of the key points:

• Purpose: CST assesses fetal heart rate response to contractions, predicting the fetus's ability to tolerate labor, especially in high-risk settings 1 2 11.
• Test Details: The test can be safely performed using either oxytocin infusion or non-invasive nipple/breast stimulation, with the latter offering increased comfort and efficiency 4 5 8 10.
• Results & Follow-Up: Results are categorized as negative, positive, or suspicious, directly influencing ongoing care. A negative result is highly reassuring, while positive or equivocal results often prompt further action 1 2 8 9 10 12.
• Clinical Impact: CST helps reduce perinatal morbidity and mortality by allowing early identification and intervention in at-risk pregnancies 2 11.
• Patient Experience: Non-invasive techniques have made the test more accessible and comfortable for patients, supporting its continued use in modern obstetric care 4 5 8 10.

By integrating CST appropriately with other antenatal tests and clinical judgment, care providers can optimize outcomes for both mothers and babies in complex pregnancies.