Contraction stress test (CST)
A contraction stress test (CST) measures the fetus's ability to tolerate the stress of uterine contractions started (induced) before true labor begins.
Electronic fetal monitoring is done during a contraction stress test to evaluate the fetus's heart rate during contractions. The fetus's heart rate during contractions helps evaluate the placenta's ability to provide enough oxygen to the fetus.
A contraction stress test that shows the placenta is not supplying enough oxygen to the fetus will help the doctor determine the safest method of delivery.
A contraction stress test is also called an oxytocin challenge test.
Why is a contraction stress test done?
A contraction stress test is done to:
- Find out if your baby will stay healthy during labor, when contractions reduce the oxygen levels.
- Check to see if the placenta is healthy and can support your baby.
This test may be done when results from a nonstress test or a biophysical profile are not normal.
How is a contraction stress test done?
A contraction stress test may be done in your doctor's office or a hospital. It is done by a family medicine doctor or an obstetrician and a trained lab technician or nurse. You probably won't need to stay overnight.
During the test, you will lie on a bed with your back raised. You will be tilted a little to your left side. This is so you will not put pressure on the blood vessels in your belly. Two belts with sensors will be placed around your belly. One belt holds the sensor that records your baby's heart rate. The other sensor measures your contractions. Gel may be used on your skin with the heart rate sensors. The sensors are hooked to a recording unit. The heart rate monitor may be moved if your baby changes position.
Your baby's heart rate and your contractions are recorded. Your blood pressure and other vital signs are also recorded.
You may be given the hormone oxytocin in a vein (intravenously, or I.V.). It is started at a low dose. The dose is increased until you have three contractions within 10 minutes that each last longer than 45 seconds. Or you may be asked to massage one of your nipples by hand to start contractions. If you don't have a second contraction within 2 minutes of the first, you will rub your nipple again. If contractions do not occur within 15 minutes, you will massage both nipples.
After the test, you will be watched until your contractions stop or slow down to what they were before the test.
How do you prepare for a contraction stress test?
Empty your bladder before the test.
If you smoke, stop for 2 hours before the test. Smoking can lower your baby's activity and heart rate.
You may be asked to sign a consent form that says you understand the risks of the test and agree to have it done.
What do the results of a contraction stress test mean?
Results of the test help predict if your baby will stay healthy during labor for the upcoming week. The test may need to be done more than once during your pregnancy.
| Normal: | Normal test results are called negative.Your baby's heart rate does not get slower (decelerate) and stay slow after the contraction (late decelerations).Your baby is expected to be able to handle the stress of labor if there are no late decelerations in your baby's heart rate during three contractions in a 10-minute period. |
| Abnormal: | Abnormal test results are called positive.Your baby's heart rate gets slower (decelerates) and stays slow after the contraction (late decelerations). This happens on more than half of the contractions.Late decelerations mean that your baby might have problems during normal labor. |
A contraction stress test may show that your baby's heart rate slows down (decelerates) when your baby is not actually having problems. This is called a false-positive result.
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