Contraction Stress Test (CST) : Purpose, Procedure, Risks and Results

Overview

Doctors perform a contraction stress test (CST) for pregnant women to examine the baby for signs of stress during uterine contractions. During this test, the doctor induces mild contractions to monitor the strength of the contractions and the baby’s heartbeat.

This blog gives a comprehensive understanding of a contraction stress test, its purpose, the procedure, and the results.

What is a contraction stress test?

Doctors perform CST on pregnant women to measure the baby’s heart rate during labour contractions. In this test, the doctor gives the pregnant woman a hormone medication known as oxytocin to contract the uterus.

These contractions are identical to labour contractions. However, they usually don’t start labour.

During labour contractions, the baby’s blood and oxygen supply temporarily drop. Most babies can handle this drop, but the heart rates of some babies are lower even after the contraction ends. This test examines the baby’s tolerance level to the decrease in blood and oxygen.

How does a CST work?

As mentioned above, the doctor gives an expectant mother a hormone medication known as oxytocin that causes their uterus to contract to carefully monitor the baby’s heart rate during and between contractions.

If the baby’s heart rate slows after a contraction, the baby could be at risk of developing issues during labour contractions.

Who can take the CST?

A pregnant woman will require only a CST if there are atypical results after biophysical profile or a non stress test:

• Non Stress Tests: In this test the heart rate and oxygen supply of your baby are checked. Your doctor or healthcare provide performs non-stress tests in about 28 weeks of pregnancy.
• Biophysical profiles: This involves a non stress test along with ultrasound (sound wave imaging) to look at your baby’s muscles, breathing, heart rate and movements.

When should you seek a medical expert?

If pregnant women experience contractions before the due date or develop any health issues, it is vital to seek immediate medical help.

Request an appointment at Apollo Hospitals

How should women prepare for a CST? 

Before the test, the doctors may provide specific instructions . Some of the instructions include:

• Fasting a few hours before the test

What can women expect during a CST?

CSTs are usually outpatient tests. Therefore, women can go home the same day after the test. During a CST:

1. Women lie on the back in an elevated position with two sensor belts wrapped around their abdomen. One belt measures the baby’s heart rate, while the other records the women’s contractions.
2. The patient receives oxytocin through an intravenous line. Doctors may also recommend another natural way to increase oxytocin – through nipple massages – that helps start contractions.
3. The doctor records the heart rate of the baby during contractions.
4. The nurse removes the IV to slow down or stop contractions.

The healthcare providers may observe the patients for complications until the end of the contraction. 

What are the risks of a CST?

CSTs are usually safe. The most significant risk of the test is that it can cause women to go into early labour.

What do the results of a CST indicate?

If the baby’s heart rate drops during contractions, it may be a sign of health issues. The doctor can recommend further testing. On some occasions, early delivery is the best option.

It is a good sign if the baby’s heart rate stays normal during the test. 

A contraction stress test may reveal whether the heart rate of your baby slows down:

1. Normal (negative) results shows that your baby’s heart rate does not stay slow after late deceleration’s (a contraction).
2. Abnormal (positive) results reveal that your baby’s heart rate slows down and stays slow after a contraction.

What happens after a positive CST? 

When the test results are positive, the baby may be unable to tolerate the stress of labour contractions. The doctor can suggest additional tests, including another stress test after a couple of weeks. If women continue to have positive results, the doctor can offer delivery through a cesarean section.

Conclusion

A CST records the baby’s heart rate during uterine contractions. The baby’s heart rate and oxygen supply are temporarily lower during contractions. Most babies can handle this temporary drop. However, if the baby’s heart rate stays low after the contraction, it could be a sign that the baby cannot take the stress of labour contractions. The doctor may recommend additional tests or a cesarean delivery when the women’s test results are positive.

Frequently Asked Questions (FAQs) 

How long does it take to perform a CST? 

Performing a CST often takes up to two hours.

What is the difference between a non-stress test and a CST?

The doctors perform a non-stress test to monitor the baby’s heart rate without an external stressors. On the other hand, a CST measures the baby’s heart rate during the stress of uterine contractions.

When is a CST performed?

The doctor usually performs this procedure on pregnant women at 34 weeks or above.

Who should not have a CST (contraction stress test)?

Women who are pregnant with multiple babies (twins, triplets or more) may not be candidates for CST (contraction stress tests). If you are pregnant with multiples, this test is more likely to induce labour.