Are you in the last few weeks of pregnancy? Did your doctor do a nonstress test for you? Sometimes, based on the results of that test, another test is needed to see if the baby can withstand the contractions that occur during labor. That test is called the Contraction Stress Test (CST). Even if you feel a little scared when you hear this name, don't worry. Today, we will talk very simply about what this does, why it is done, and how.
What is Contraction Stress Test (CST)?
Simply put, a Contraction Stress Test (CST) is a test that checks whether your baby can withstand the pressure of uterine contractions during labor while you are pregnant.
Think of it this way: childbirth is like a mini-marathon for the baby. This test is used to determine in advance whether the baby has the strength to run that marathon and whether it can complete it without any problems.
When the uterus contracts during labor, the baby's blood and oxygen supply temporarily decreases. This is normal. Most healthy babies can handle this temporary decrease without any problems. However, some babies' heart rates remain low even after the contractions have ended. This CST is used to detect such a risk early.
In this test, your doctor gives you a hormone to make your uterus contract. These are similar to the pains you experience during labor, but they don't usually start labor. By watching how your baby's heart responds to these artificial contractions, doctors can determine whether your baby will be able to handle the real labor.
Who wants to take this test?
This CST test is not recommended for every pregnant woman. It is usually only recommended if you have noticed any abnormalities in the results of another test you have previously done, such as a nonstress test or a biophysical profile .
- Nonstress Test (NST): This is done to check the baby's heart rate and oxygen levels while you are still, without any pressure. This is done around 28 weeks into your pregnancy.
- Biophysical Profile (BPP): In this, along with the nonstress test, an ultrasound scan is used to check the baby's heartbeat, breathing, muscles, and movements.
If the doctor has doubts about the results of either of these two tests, he or she will refer the baby for a CST to further confirm the baby's condition.
What is the difference between a nonstress test and a contraction stress test?
Although the names of both these tests may sound similar, there are clear differences between the two. Let's see what they are.
| Characteristic of the test | Nonstress Test (NST) | Contraction Stress Test (CST) |
|---|---|---|
| Purpose of the test | Check the baby's heartbeat when he is relaxed and without any pressure. | To see how the baby's heartbeat responds to the pressure of uterine contractions. |
| How to do it | While you relax, you monitor the baby's heartbeat and movements. You don't do anything extraneous. | Uterine contractions are artificially induced by administering a hormone (oxytocin). |
| The effect on the baby | The baby is not put under any stress. That's why the name says "Nonstress". | Just like during childbirth, controlled stress is applied to the baby. |
Simply put, the NST checks to see if the baby is doing well while just sitting still. The CST checks to see if the baby is doing well while "doing something" (i.e. during labor).
What happens when you take this test?
This is usually a test done in the outpatient department of a hospital. This means you can go home after the test. This can usually take about two hours.
How should you prepare before the test?
Your doctor will give you specific instructions about this. Usually,
- You will be asked to refrain from eating or drinking for 4 to 8 hours before the test.
- If you are a smoker , you should stop smoking at least two hours before the test, as smoking can slow down the baby's heart rate and may cause incorrect test results.
During the test...
1. You will be made to lie down on a bed, leaning back slightly.
2. A nurse will come and tie two straps with sensors around your belly. One sensor will measure the baby's heartbeat, and the other will record your uterine contractions.
3. Then, a cannula is inserted into a vein in your arm, like a saline drip, and a hormone called ``Oxytocin`` is slowly injected into your body through it. This hormone helps the uterus contract. Sometimes, you may be asked to massage your nipples to stimulate the natural production of oxytocin.
4. When the uterus starts to contract, the machine records the baby's heartbeat at that time and during the time it is in labor. The doctor carefully monitors this.
5. Once the test has collected enough data, the nurse will come and stop giving you the oxytocin. After a while, the contractions will stop.
After the test is over, the medical team will monitor you until your contractions have completely stopped. Sometimes, if the contractions don't stop, they may give you medicine to stop them.
Is there any risk in this?
CST is generally a very safe test. However, the biggest risk is that these artificially induced contractions can sometimes cause labor to begin before the due date. This is very rare, but it is important to be aware of the risk.
Also, if you are expecting twins or triplets , you may not be eligible for this test. This is because the risk of inducing labor is higher with multiples. This is all determined by your doctor after talking to you.
What do the test results say?
The results of this test, which takes about two hours, indicate whether the baby's heart rate slows down when the uterus contracts, and if so, how long it lasts.
| Result | Simple meaning |
|---|---|
| Normal (Negative) | This is the best outcome . It means that the baby's heart rate has not slowed down after the uterine contractions have finished. It means that the baby is able to withstand the pressure of labor well. Doctors call this condition 'late decelerations' . |
| Abnormal (Positive) | This means that the baby's heart rate has slowed even after the uterus has finished contracting. This is a sign that the baby may be having difficulty coping with the pressure of labor. |
What do you do next if the result is positive?
Don't worry if the result is positive. This means that the baby may experience some discomfort during delivery, so we need to be extra careful about that.
Your doctor, in a situation like this,
- It may be recommended to do some more tests.
- You may decide to do another CST in a week or two.
- If the results continue to be positive, and there is a concern about the baby's safety, a cesarean section may be recommended, as it is dangerous to put the baby through the strain of labor.
In this case, your doctor will make a decision after talking to you and explaining what the safest option is for you and your baby.
This CST test measures the baby's health at that moment. Therefore, mothers with certain risk factors may need to have this test done once a week during the last few weeks of pregnancy.
Take-Home Message
- The Contraction Stress Test (CST) is a safe test that checks whether the baby can withstand the pressure of childbirth.
- This is usually only done if there is any doubt about the results of a Nonstress Test (NST).
- If the test result is 'Negative', it means the baby is fine. That's good news.
- If the result is 'Positive', it means that the baby is likely to have some difficulty during delivery. In such a case, your doctor will advise you on the best next step, which may include a cesarean section.
- Talk openly with your doctor about any questions or concerns you have about this test or the results. Don't hold anything back.
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