What Is A Contraction Stress Test (CST) And How Is It Done?

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In the case of high-risk pregnancies, it becomes essential to check how the baby will respond to the contractions during childbirth. To determine how the fetus will cope, doctors will put you through a special pregnancy test called the contraction stress test.

MomJunction helps you understand the significance of this test during pregnancy and how and when it becomes necessary.

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.

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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).

A contractions stress test is needed especially in high-risk pregnancies with complications such as (3, 4):

  • 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
  • Stillbirth
  • 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.

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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.

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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.

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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.

  • Equivocal-suspicious

Intermittent decelerations or a significant variance in the decelerations is noticed. This ambiguity calls for a repeat test.

  • Equivocal-tachysystole

There is one contraction in more than every two minutes, which lasts for 90 seconds, indicating fetal heart rate decelerations.

  • Equivocal-unsatisfactory

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.

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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.

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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.
  • Obesity.
  • 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.

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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.

Pregnancy is a delicate phase in life. The well-being of the mom and baby should be regularly monitored, particularly when there is any risk associated with the pregnancy. Do not miss any prenatal checkups, and talk to your doctor to keep a check on how the pregnancy is progressing. Get tested as and when required to ensure you and your baby are safe through the term.

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Did you go through a contraction stress test during pregnancy? Share your experience with us in the comment section below.