The Contraction Stress Test (CST) is used to determine whether the fetus in the stomach can still be contracted. To stay healthy, this test includes in vitro heart tone monitoring, and conducted to usually over 32 weeks of pregnancy. When the uterine contracts, the amount of blood and oxygen to the fetus decreases for a short period of time. Although this is not a problem for most fetuses, the heart rate of some fetuses can be slowed down.

In prenatal stress tests, intravenous oxytocin (a hormone) can cause contraction of the uterine. In addition, it can massage the breast to release oxytocin if the fetus is contracted. After the heart beats slower rather than faster, the fetus is likely to not be able to withstand the stress of natural contractions.

Why is a Contraction Stress Test required?

Pregnant women often perform a nonstress test or a biophysical profile to observe the condition of the fetus. If the results are abnormal, your doctor may recommend further stress testing to confirm the following:

  • Check whether the fetus in the belly can remain healthy in the event of contractions in childbirth
  • The placenta is healthy enough to support the fetus.

Before testing

Prenatal stress tests may show a slow heart rate even when the fetus is not having problems, which is called false positive results. Prenatal stress tests are rarely performed for a variety of reasons, and in most cases physicians can assess the condition of the fetus faster and safer through fetal physiological and non-stress tests.

Knowledge of the Test Process

What should I prepared in advance?

  • You may be asked not to eat or drink for 4 to 8 hours before the test.
  • Please keep the bladder empty before the test.
  • Do not smoke 2 hours before the test if you have a habit of smoking, as smoking will reduce fetal activity and heartbeat.
  • To sign a consent form to indicate understanding of the test risks and consent to the conduct of the test.

If you have any concerns about the test, consult your physician, e.g. test risk, test process, or test results.

What are the test steps of Contraction Stress Test?

You may be asked not to eat or drink for 6 to 8 hours before the test, as there is still a small chance that acute Caesarean section is needed.
At the beginning of the test, the subject needs to lie on the left side. Medical staff will attach two devices to the stomach, one to monitor the heart of the fetus and the other to record the contraction of the uterus. The instrument records contractions and fetal heart sounds, and the result is presented as 2 different lines on the chart.

This test continues until 3 contractions every 10 minutes, each of which takes about 40 to 60 seconds, which may take up to 2 hours as they  may not feel contractions.
If you fail to construct in your uterus for the first 15 minutes, medical staff may cause contractions by injecting low doses of oxytocin, or ask the subject to massage the breast so that the body naturally releases oxytocin.

Post-test information

A prenatal stress test may take up to 2 hours. After the test, the doctor will ensure that contractions stop, slow down, or are similar to the pre-test contractions. If you have any questions about your prenatal stress test, consult your doctor for best assistance.

Results after testing

The results of the test can represent the health of the fetus for one week and may be performed more than one test during pregnancy.


Fetal heart rate has not slowed down or delayed slowdown after contraction of the uterus. Perhaps the fetal heart rate has slowed down several times during the test, but as long as it is not maintained, it is still indicating normal.


In more than half of the uterine contractions, there is a slowdown in fetal heart rate or delayed slowdown, which may indicate that the fetus cannot withstand natural uterine compression.Thus, requiring special attention.

What factors affect test results?

The following are the reasons why pregnant women may not be able to take the test and may cause the test results to be inaccurate:

  • Problems in past pregnancy, such as vertical incision during caesarean section, pre-placenta or early placenta peeling.
  • If you have more than one fetus
  • If you have Premature Rupture of Membrane
  • Uterine surgery has been performed in the past because strong contractions can lead to a rupture of the uterus.
  • Smoking or taking drugs (illegal substance)
  • The movement of the fetus during the test may make it difficult for the sensor to measure the fetus’s heart rate or contraction of the uterus.
  • Excessive weight

Different laboratories and hospitals have different prenatal stress test results. Consult your doctor if you have any questions about the test results.

Read: Week 42 of Pregnancy: Over Your Expectations