The contraction stress test or CST blood test is a method of predicting the baby’s behavior during labor. This test uses artificial contractions to understand how a baby would respond to natural labor contractions by studying their heart rate. This test is mostly performed on high-risk pregnancies. If your doctor has suggested a CST, then here is all you need to know about it. Read on to know more about CST, its method, the risks, and the contradictions in this post.
What Is A Contraction Stress Test (CST)?
CST measures the fetal heart rate during late pregnancy or labor. The purpose of this test is to induce contractions and ensure that the baby is getting sufficient supply of oxygen from the placenta during uterine contractions. This test is also known as the Stress test or the Oxytocin challenge test.
Women with pregnancy complications are generally subjected to this test at around 34 weeks or later (1). Moreover, the number of tests a pregnant woman may need depends upon the extent of risk in her pregnancy.
CST is recommended if the doctor thinks your pregnancy poses risks, especially to the baby.
Why Is CST Done?
CST is done to determine the baby’s well-being during contractions or labor and to check if the placenta is adequately supporting the fetus. If the placenta has enough oxygen and blood for the fetus, then the heartbeat will not be affected.
The average fetal heart rate is 110-160 beats/minute and can fluctuate based on the uterine conditions. Any abnormality in the fetal heart rate indicates an insufficient supply of oxygen or some other fetal distress (2). Usually, the uterine contractions decrease the flow of blood or oxygen to the fetus through the placenta. This could bring a variation in the fetal heart rate.
The contraction stress test is recommended when the results of a non-stress test or biophysical profile are abnormal (1).
- Heart disease
- Type-1 diabetes
- Rh-factor sensitization
- Poorly controlled hyperthyroidism
- Antiphospholipid syndrome
- Systemic lupus erythematosus
- Hypertensive disorders
- Hemoglobinopathies such as hemoglobin SS, SC or S-thalassemia
- Chronic renal disease
- Intrauterine growth restriction
- High BP
- Postterm pregnancy
- Decreased fetal movements
- Multiple (twin or triplet) pregnancy with growth discrepancy
- Oligohydramnios, or deficiency of amniotic fluid
- Polyhydramnios, or excess amniotic fluid
The following section details how the test is carried out at the hospital.
How Is The Contraction Stress Test Done?
Before the test, you are told not to eat or drink anything for six to eight hours. Also, the patient needs to empty her bladder before the procedure starts.
The test: You have to lie to your left side, and two devices are strapped around your belly. One device monitors the fetal heartbeat and the other records the uterine contractions. The heartbeat and the contractions are recorded on a graph paper to evaluate the result.
The test lasts until you get three contractions (each lasting for 40 seconds) within ten minutes (5). The contractions that you feel may be mild, similar to menstrual cramps.
- Oxytocin challenge test: If the contractions do not start on their own within 15 minutes, then a small dose of oxytocin (Pitocin) is given to you through IV.
- Nipple stimulation: It is another method to kick-start the contractions. The doctor may ask you to stimulate the nipples to release oxytocin in the body and trigger contractions naturally.
After the test, you need to wait until the contractions get back to normal. So the overall time taken for the test is around two hours.
How Does It Feel During The Contraction Stress Test?
Lying on your left side may not be comfortable for you during the contractions. Moreover, the electronic sensors around your belly may make you feel a bit uncomfortable. Most women who have undergone this test have said that it is a painless but uncomfortable procedure (1).
Keep reading to know how the doctors interpret the CST results.
How Is The CST Result Interpreted?
The contraction stress test results are obtained immediately from the graphical record in the electronic device used for fetal monitoring during the test (4). Results could be:
- Negative contraction stress test
The CST result is normal/negative when there are no decelerations in the fetal heart during the contractions, recorded at the rate of 3 contractions/10 minutes.
- Positive contraction stress test
The test result is positive if more than half of the contractions indicate late decelerations in the fetal heart rate, recorded at the rate of 3 contractions/10 minutes.
Intermittent decelerations or a significant variance in the decelerations is noticed. This ambiguity calls for a repeat test.
There is one contraction in more than every two minutes, which lasts for 90 seconds, indicating fetal heart rate decelerations.
The result is unsatisfactory when fewer than three contractions occur in 10 minutes or when the tracing cannot be interpreted properly.
Like a few other tests, this test may also have some risks.
What Are The Risks Of This Procedure?
Some of the risks of CST are: (1):
- The use of oxytocin could lead to early labor. It may also prolong the contractions, but they can be stopped with the help of medicines. If the contractions do not subside, an emergency delivery will be recommended by the doctor.
- The fetal heart rate monitoring device may be faulty and indicate health problems in your baby even though the baby is doing good.
- False positive contraction stress test could happen when the contraction stress test shows a drop in the fetal heartbeat when your baby is healthy. The test result could also be falsely positive in the case of patients with hypertension.
Though the test is done in the case of high-risk pregnancies, it is not recommended for a certain group of pregnant women.
What Are The Contraindications For This Test?
Contraction stress test might not be viable in some cases.
- Women carrying multiples, because this test could stimulate preterm labor.
- A patient with a ruptured membrane from a previous pregnancy or uterine surgeries.
- Any signs of preterm labor.
- Patient with a previous classical c-section.
- Placenta abruption or placenta previa.
- Patient with an incompetent cervix.
- Gestation age is less than 32 weeks.
- If the baby’s movements are more during the test, it becomes difficult for the device to record the heartbeat or contractions.
Next, we understand how the contraction stress test is different from the non-stress test.
CST Vs. NST
Here are the differences between the non-stress test (NST) and contraction stress test (CST) (6):
|The fetal heart rate is recorded in response to the uterine contractions.||Fetal heart rate is measured in response to fetal movement, without putting any stress on the fetus.|
|Nipple stimulation or oxytocin is used to start a uterine contraction.||Uterine contractions are not triggered in this procedure.|
|The test results are read as negative (normal) or positive (abnormal).||The test results are read as reactive (normal) or non-reactive (abnormal).|
|It is expensive and cumbersome.||It is cost-effective, quick, and safer.|
|Not commonly used nowadays.||Preferably recommended by doctors nowadays.|
Frequently Asked Questions
1. What happens if a baby fails a stress test?
“Failing” a contraction stress test corresponds to getting a positive result on the test. In such a case, late decelerations in heart rate (slowed heart rate) are observed in more than half of the contractions (4). It may indicate insufficient oxygen supply or other complications.
2. Does everyone require a non-stress test during pregnancy?
Not all pregnant women require a non-stress test. It is suggested in case of underlying chronic health issues, a history of complicated pregnancies, multiple pregnancies, slow fetal movements, and other indicators of probable pregnancy complications (6).
3. Do Braxton Hicks show on NST?
No, laboratory or radiographic tests cannot diagnose Braxton Hicks contractions. These contractions are evaluated by assessing the pregnant woman’s abdomen, specifically by palpating the contractions (7).
4. Can I eat before a nonstress test in pregnancy?
Yes. In fact, doctors may suggest you eat something before the test since babies usually show increased movement after their mothers have eaten (8).
CST test during pregnancy helps determine if the baby is getting sufficient blood and oxygen during contractions. This test is advised around 34 weeks of pregnancy if you have abnormal non-stress test results. It is also recommended for women with pregnancy complications such as heart disease and type-1 diabetes. The CST test can be safely used in most cases. However, it isn’t viable if the expecting mother has placental abruption or placenta pervia, incompetent cervix, or a gestational age of lesser than 32 weeks.
- Contraction Stress Test.
- Non-stress/contraction Stress Tests (fetal heart rate testing).
- Monica Preboth; ACOG Guidelines on Antepartum Fetal Surveillance; American Academy of Family Physicians (2000).
- Contraction Stress Test.
- Lawrence D. Devoe MD; Nonstress and Contraction Stress Testing; The Global Library of Women’s Medicine.
- Nonstress Test.
- Deborah A. Raines and Danielle B. Cooper; (2021); Braxton Hicks Contractions.
- Nonstress Test.