A contraction stress test (CST) is performed during pregnancy to check the health and control the blood supply to the fetus. This test is difficult, invasive, and expensive. In high-risk pregnancies, it is necessary to check how the fetus responds to contractions during delivery. To determine the reaction of the fetus to these conditions, doctors perform the CST in pregnancy.
What is the Contraction Stress Test (CST) in Pregnancy?
A contraction stress test (CST) - also known as a stress test or oxytocin challenge test (OCT) - is done to measure fetal heart rate in the course of uterine contractions during pregnancy. The purpose of the test is to check the oxygen supply to the fetus through the placenta during delivery.
Due to various reasons, the contractile stress test (CST) is rarely used in pregnancy today. In most of the cases, doctors can assess the condition of the fetus more safely and quickly by using test or biophysical profile (BPP) or a nonstress test (NST), or both of them.
During contractions, blood and oxygen flow to the placenta is temporarily reduced. If the placenta is good and healthy, oxygen is supplied to the fetus during contractions with the help of the resources stored in the placenta. Therefore, if everything goes well, the fetal heart rate will not decrease during or after contractions. But if the placenta does not work properly, the fetus will not receive enough amount of oxygen and its heart rate will decrease.
Contraction stress testing is a difficult, expensive and dangerous test, which is why it is rarely performed today. But if you have a high-risk pregnancy, the doctor may suggest doing this test near the date of delivery.
How is the Contraction Stress Test (CST) done during Pregnancy?
To perform this test, you should avoid eating or drinking for 6 to 8 hrs. before the test. Because the result of the test may lead to a c-section, it is better to empty your bladder before doing the test.
During the test, you must lie on your left side. The technician ties two devices using straps to the abdomen: one monitors the fetal heart rate and the other one records uterine contractions. A machine also prints the chart of fetal contractions and ECG as two distinct lines on paper.
This test continues until you have three contractions within 10 minutes, each contraction lasting for 40 to 60 secs. The testing process may take up to two hrs. The contractions created are of low intensity and are hardly felt.
You may feel pain similar to menstrual cramps during contractions. If the contractions are strong, they may lead to labor induction.
If you don't have any contractions in the first 15 mins of the test, the doctor will inject a little synthetic oxytocin (known as Pitocin) or ask you to stimulate your nipples. This will release natural oxytocin into the body.
Once the test is over, you will be monitored until the contractions stop or return to pre-test levels.
How are the Test Results Interpreted?
If the fetal heart rate does not decrease in response to contractions, it indicates that the placenta is functioning well. This result is reported as a normal or negative contraction stress test result. In such situations, you should wait until the time of natural delivery or repeat the test a week later.
If the fetal heart rate decreases in response to more than half of the contractions, it is a positive contraction stress test and indicates that the fetus is under pressure. In this case, the doctor may decide to induce labor. If the fetus is unable to tolerate contractions or is at risk of premature labor, the doctor will request a c-section.
Interpretation of the reported results
The negative and positive results of the contraction stress test are immediately obtained from the graphical record output from the device. The results are reported as follows:
Negative contraction stress test
The result of CST is normal/negative if there is no drop monitored in fetal heart rate during contractions (at a rate of 3 contractions in 10 minutes).
Positive contraction stress test
The test result is positive if more than half of the contractions show a decrease in the fetal heart rate (at a rate of 3 contractions in 10 minutes).
A drop in the heart rate or the presence of significant variance in the data causes uncertainty in the interpretation of the results. This ambiguity requires repeating the test.
In this condition, there is a contraction every two minutes that lasts 90 seconds. This result indicates a decrease in fetal heart rate.
This result is not satisfactory. This result is reported if fewer than three contractions occur in 10 minutes or when the results cannot be properly interpreted.
However, take into consideration that as with other tests, this test may have errors.
Pregnancy is an enjoyable stage in a woman’s life. The health of the mother and fetus during this period should be monitored regularly, especially when the risk of pregnancy increases. Carry out pregnancy care checkups regularly and talk to the doctor about the development of the fetus and its health. If prescribed by the doctor, perform the contraction stress test calmly so that you can rest assured about the health of the fetus.