Contraction Stress Test (CST): Purpose, Procedure, Interpretation & Results

Antenatal (before birth) tests are useful in detecting any health concerns during pregnancy for both the baby and the mother. Early diagnosis and prompt treatment plans can be lifesaving in many cases. The contraction stress test (CST) is a test that is done during late pregnancy. This article explains in detail the purpose, procedure, and results of the contraction stress test.

What is a Contraction Stress Test (CST)?

The contraction stress test is performed to assess how the baby's heart rate and oxygenation levels respond to labor contractions. It is normal to have a slight drop in the heart rate and oxygenation level of the baby during labor, but a significant drop or a persistent lowering of these parameters indicates that it may not be safe for the baby to be delivered through normal vaginal delivery.

When is a Contraction Stress Test Recommended?

The contraction stress test is not performed very commonly. It is not a routine prenatal test. CST is performed in complicated pregnancies. Initially, your doctor will do a biophysical profile or nonstress test. These tests check the heart rate, oxygen supply, and muscle movement of the baby under normal, non stressful conditions and are performed at about 28 weeks of gestation. If the results of these tests turn out abnormal, your doctor may want you to undergo a contraction stress test at 32 weeks or later. You may need to repeat CST a few times, depending on the results.

What is the Principle Behind a Contraction Stress Test?

The contraction stress test utilises the fact that the hormone oxytocin causes uterine contractions. To perform CST, your doctor injects oxytocin to induce the contractions and mimic the labor environment so that they can know the status of the baby’s heart rate and oxygen saturation during labor. The results help them understand the scenario beforehand, be better prepared, and take an appropriate decision regarding labor and delivery before the actual labor induction.

How is the Contraction Stress Test Performed?

The contraction stress test is usually performed as an outpatient test, which means you do need to get admitted for this test.

The test takes about 2 hours. It is performed in the following way:

1. You are asked to lie on the bed.
2. Two belts that have sensors attached to them are wrapped around the belly. The first one measures the heart rate of the baby, while the second monitors your uterine contractions.
3. You will be administered oxytocin intravenously to induce contractions. Because rubbing nipples promotes oxytocin secretion by the body, your doctor may advise you to do so.
4. The baby's heart rate and oxygenation levels are recorded during the contractions.
5. After enough data has been obtained, Oxytocin IV is discontinued to stop the contractions.
6. You will then be kept under observation until the contractions cease. If they do not stop soon, you may be given medication.

Are there Any Risks Associated with the Contraction Stress Test?

The contraction stress test is usually safe. The only risk is that it might result in early labor induction in a few cases, causing preterm delivery.

Because the risk is higher in multiple pregnancies (twins or triplets), it is rarely used in such cases.

Contraction stress test risks are higher in pregnant women with conditions such as placenta previa, risk of uterine rupture, or a history of previous caesarean section.

Who Should Not Have a Contraction Stress Test?

Women pregnant with twins or triplets (multiple pregnancies) are not considered ideal candidates for a contraction stress test because they have a higher risk of going into labor during CST. Your doctor, on the other hand, will discuss and weigh the potential risks and benefits with you in order to make an informed decision.

Is There Any Preparation Required for the Contraction Stress Test?

Follow the instructions given by your doctor regarding preparation before undergoing a contraction stress test. You may be asked not to eat or drink for 4 to 8 hours before the test.

Additionally, if you are a smoker, you must not smoke for at least 2 hours before the test, as smoking can lower the baby's heart rate and give erroneous results.

Expect some discomfort during CST. Mentally preparing yourself beforehand will be helpful.

How to Interpret the Results of a Contraction Stress Test?

The contraction stress test results are interpreted as below:

Normal/Negative result: A normal result indicates that the baby’s heart rate does not remain slow after the contractions; thus, the baby will not have this medical concern during labor and delivery. You may be asked to repeat the test after a week or so even if you had a negative CST result, but other tests show some indication of possible issues.

Abnormal/Positive result: Positive contraction stress test indicates that the baby’s heart rate lowers down and remains slow during contractions. This can be a cause of concern during labor and delivery.

What to Do if You Have Abnormal Contraction Stress Test Results?

Abnormal contraction stress test results imply that it might be difficult for your baby’s heart to bear the stress of uterine contractions during labor. Delivery through a caesarean section is generally suggested in such cases. Early delivery may also be recommended.

Do not panic if your CST results are positive. More than being a diagnostic test, CST is used as a tool to rule out certain medical conditions. It is not uncommon for women to have healthy babies despite a positive contraction stress test.

Bottomline

The contraction stress test (CST) is a very beneficial antenatal test that helps anticipate the tolerability of the baby for the stress it is likely to encounter during labor. If found inadequate, it gives the healthcare provider and parents ample time to decide upon the most optimal course of action and minimizes the risk factor. CST, thus, plays a vital role in reducing childbirth complications and can be lifesaving.