The contraction stress test (CST) - also called a stress test or an oxytocin challenge test - may be done during pregnancy to measure a baby's heart rate during uterine contractions. It's not often used, but some healthcare practitioners will do one when the decision to deliver has been made but they aren't sure if the baby is strong enough to tolerate labor. Also, if you're having a high-risk pregnancy, it's possible that your practitioner would recommend a contraction stress test as you get close to your due date.
Here's how it works: During contractions, the flow of blood and oxygen to the placenta temporarily slows down. If your placenta is healthy, it has extra stores of blood ready to provide the baby with the oxygen he needs during contractions. So if everything is okay, your baby's heart won't slow down during or after a contraction. But if the placenta isn't functioning properly, your baby won't get enough oxygen and his heart will beat more slowly after a contraction.
Why isn't it used often?
When it's time, most healthcare practitioners will make the decision about how to deliver your baby right away, without added testing. Or the baby will make the decision, once labor begins (monitoring him during labor will tell whether or not he's tolerating the contractions).
If testing is needed (your doctor wants to see how the baby is doing at the end of your pregnancy, before it's time to deliver, for example), the biophysical profile or nonstress test or both are usually considered better choices. That's because these tests are safer (less likely to trigger labor), quicker and easier (no need for an IV) and more accurate (see false positive rate, below) than the contraction stress test.
What's the procedure like?
You'll be asked not to eat or drink anything for six to eight hours before the test, on the slim chance that the results will call for an emergency c-section. (Emptying your bladder shortly before the test is also a good idea, because the test can take a while.)
When it's time for the test:
What do the results mean?
A negative result
If your baby's heartbeat doesn't slow down in response to your contractions, he's probably doing fine. This is called a normal or negative result. In this case, you'll either wait to go into labor naturally or have the test again in a week.
A positive result
If your baby's heart beats more slowly after more than half of your contractions, the test result is positive, signaling that your baby may be under stress and unable to tolerate labor contractions. In that case, your practitioner might recommend a cesarean section right away or continued observation.
The stress test is very reliable when it indicates that everything is okay, but not so reliable when it indicates that there might be a problem. Your chances of getting a false positive result (indicating a problem when there isn't one) could be as high as 30 percent.
This might mean that your baby's heart reacted with more frequent or longer-lasting contractions, or that it slowed intermittently.
Are there any risks from this procedure?
There's a small risk that your uterus will be hyperstimulated, causing contractions so strong and frequent that they cut off blood flow to the baby. (This may be more of a risk from nipple stimulation, since it's harder to control than oxytocin, but can be caused by either.)
The test may also stimulate premature labor. This can happen if you get too much oxytocin too quickly (sensitivity to the drug can vary from woman to woman). That's why the test is not recommended if you have any risk factors preterm labor.
Again, because of these risks, and the fact that the test is not reliable in most situations, most practitioners will avoid the contraction stress test if possible and will recommend a less invasive or complicated procedure. Read about those other options in our articles on the biophysical profile and the nonstress test.
The contraction stress test (CST) is a test to see how your baby might cope with contractions during labor. If recommended, the test is typically done near the end of pregnancy.
To perform the test, your doctor will induce mild contractions. An external fetal monitor, which is strapped to your abdomen during the test, keeps track of the strength of the contractions and your baby’s heartbeat.
Contraction stress tests (CST) are one way to monitor the health of a fetus before labor. A CST may be part of a series of tests you undergo. Other common late-stage pregnancy monitoring includes a non-stress test (NST) and a biophysical profile (BPP).
The goal of all late-stage pregnancy tests is to assess whether a fetus is at risk for stillbirth. If the results of CSTs are not reassuring, doctors can intervene prior to labor.
Your doctor might suggest a CST if:
Since a CST can sometimes jump-start labor, it is not recommended for people who are at risk for preterm labor or for people who have placenta previa.
A CST is done near the end of pregnancy, usually at 34 weeks or later. The test is often done in the hospital.
During the test, your doctor will place two straps over your abdomen—one to measure contractions, the other to measure fetal heart tones. Your doctor will administer Pitocin (artificial oxytocin) to induce contractions. Alternately, you may stimulate your nipples by rubbing them, which can also bring on contractions.
Your doctor will be looking for a frequency of three contractions every 10 minutes. Your doctor might need to increase the Pitocin until your body responds with contractions that are frequent enough. The test itself may last a couple of hours from start to finish. After the nurse discontinues the medication, they will keep an eye on you until your contractions stop.
CST results may be positive or negative. They may also be uncertain. When test results aren’t clear, your doctor may want to repeat the test in a couple of days:
The results of the CST last for one week. If your test is negative and your pregnancy continues beyond a week since your previous CST, your doctor may want to repeat the CST in a few days to a week.
Studies have found a positive result to be a valuable predictor of abnormal fetal heart rate patterns during delivery. If the results of the CST are positive, your doctor may suggest options for induction or cesarean birth.
A CST offers valuable information, but it does carry some risks. Your doctor will discuss the risks and benefits of CST with you.
Risks of CST include:
Hospital staff will closely monitor you during and after the test. If you are at risk for preterm labor or have placenta previa, a CST may not be recommended.
If you are unable to have a CST, your doctor may use a combination of other fetal monitoring options to assess your baby’s well-being, including:
A non-stress test monitors a fetus without inducing contractions. A contraction stress test, on the other hand, monitors a fetus’ response to contractions. During a CST, mild contractions are induced either through nipple stimulation or medication.
If results are negative or inconclusive, your doctor may want to repeat the test weekly or twice a week until birth.
If you are facing a CST, you may be feeling anxious. Rest assured that these tests have been used for decades and are considered safe. The test itself usually only produces mild discomfort.
Talk to your doctor about any questions or concerns that you have about the test. Ask them about what the results could mean and what the next steps are. You may want your partner or doula to accompany you to the test for reassurance and support.
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