Contraction Stress Test

A contraction stress test (CST) checks for signs of fetal distress during stimulated uterine contractions. It’s also called an oxytocin challenge test. The test helps your healthcare provider see how the fetal heart rate reacts when your uterus contracts.

Providers rarely use CSTs anymore. They typically rely on nonstress tests (NSTs) and biophysical profiles (BPPs) to check how the fetus is doing. These tests are quicker and less invasive. But CSTs can be a tool to check for fetal well-being and to see if the fetus can handle the stress of labor. Your provider may recommend it if other test results are unclear.

You may be more likely to need a contraction stress test if you have:

• A high-risk pregnancy
• Intrauterine growth restriction
• A placental problem

Providers only do this test if you’re in your 34th week of pregnancy or later.

How does the contraction stress test work?

A contraction stress test triggers mild contractions and tracks the fetus’s heartbeat to see if it’s OK with a drop in blood and oxygen.

During contractions, it’s typical for a fetus’s blood and oxygen supply to drop temporarily. Most fetuses can handle this decrease. But some fetuses’ heart rates remain low after the contraction ends.

How do I prepare for the test?

Your healthcare provider will give you specific instructions to prepare for a contraction stress test. You may need to stop eating and drinking (fast) for four to eight hours before the test. This is in case the results show that you need a C-section right away. Your provider will let you know if this is the case.

You should not smoke. Smoking can decrease your baby’s heart rate and interfere with the test results.

How is it performed?

During a contraction stress test:

1. You’ll lie on a bed with your back slightly elevated and tilted to the side.
2. A nurse will wrap two belts with sensors around your belly. One sensor measures the fetal heart rate. The other records your contractions.
3. Your healthcare provider may ask you to massage your nipples. This helps trigger mild contractions. Or you may receive oxytocin through an IV to trigger contractions.
4. Your provider checks the fetal heart rate before, during and after contractions. You’ll need to have at least three contractions that last at least 40 seconds within 10 minutes. This can take a while to get.
5. Once the test is over, a nurse will stop the oxytocin (if you got it) and remove your IV.

The test often takes up to two hours.

What are the risks of a contraction stress test?

Contraction stress tests are typically safe. The biggest risk is that it may cause you to go into labor before your due date.

What to expect after the test?

Right after the contraction stress test, your care team will observe you until your contractions stop. If your contractions don’t stop, your provider may give you medicine to help.

What do the results of a contraction stress test mean?

A contraction stress test tells your healthcare provider if the fetus’s heart rate can tolerate contractions. You’ll get one of these results:

• A normal (negative) result means the fetal heart rate responds as it should to a contraction. It doesn’t decrease after a contraction.
• An abnormal (positive) result means the fetal heart rate slows down and stays down after a contraction (late decelerations). It happens after half or more of the contractions.

What happens after a positive contraction stress test?

If your test results are positive, it means the fetus may be unable to tolerate the stress of labor contractions.

Your healthcare provider may recommend additional tests, including another stress test a week or two later. If you continue to have positive results, your healthcare provider may recommend delivery.

When should I see my healthcare provider?

If you go into labor or your water breaks after a contraction stress test or before 37 weeks of your pregnancy, call your healthcare provider right away. This is preterm labor.