Contraction Stress Test

What is a contraction stress test?

A contraction stress test checks the baby's heart rate when the mother has a uterine contraction. During this test, the baby's heart rate and the mother's contractions are recorded.

Most contractions decrease the flow of blood and oxygen to the baby for a short time. By seeing how the baby's heart rate reacts to contractions, your healthcare provider can tell if the baby will be able to handle the stress of the contractions that occur during labor.

This test is also called an oxytocin challenge test.

A contraction stress test is usually done between the 32nd week of pregnancy, but in high risk pregnancies, may begin at 26 to 28 weeks.

When is a contraction stress test used?

This test may be done if:

• During a nonstress test the baby's heart rate did not rise enough to be considered a reactive result. (A nonstress test uses an external fetal monitor to look at how the baby's heart rate changes when the baby moves.)
• The biophysical profile score is low. (For a biophysical profile, an ultrasound scan is done with a nonstress test.)
• Your healthcare provider knows or thinks that your baby or you have a high-risk condition.

Often a biophysical profile may be done instead of a contraction stress test.

How is a contraction stress test done?

You will lie on your left side with a strap and pressure gauge around your abdomen. The gauge measures contractions of the uterus. An ultrasound transducer will be placed on your abdomen over the baby's heart. It measures your baby's heart rate. Your blood pressure and the baby's heart rate will be monitored for several minutes before the test begins.

For the test to be valid, you must have contractions that are strong and frequent enough to be similar to the first phase of labor. If you are not having contractions that are strong and frequent enough, your healthcare provider may give you a very small amount of oxytocin through a vein (IV) until you have 3 contractions, lasting 40 seconds, in 10 minutes.

How is the test interpreted?

Your provider will look at how often the baby's heart rate slows and when during a contraction it slows.

• If the baby's heart rate does not fall during the contractions, the result of the test is negative, which is reassuring. It suggests that the baby is healthy.
• If the baby's heart rate falls during a contraction, the test may be interpreted as positive and nonreassuring. This may mean that the baby may be having some problems and there is a chance the baby may have more serious problems during labor. It is important to remember that not all lowering of the heart rate during a contraction means the baby may have a problem. The test has to be read by your healthcare provider based on the situation and circumstances of the test.

What happens after a contraction stress test?

A result that is not clearly positive or negative may be repeated in 24 hours. Your healthcare provider may want to repeat the contraction stress test weekly in the case of a high risk pregnancy.

If you have a positive stress test, you may be admitted to the hospital. If your baby is having a problem that cannot be corrected, your healthcare provider may want to deliver the baby early by inducing labor, or with a cesarean section.

What are the risks of a contraction stress test?

The stress test can cause some women to start labor. For this reason it is not done if you are at risk of going into premature labor.

The test could cause too many uterine contractions, especially if you are given oxytocin to stimulate the contractions. Oxytocin is given slowly and carefully, but in some cases it can stimulate too many contractions and has to be stopped.

Your healthcare provider also may not use this test if:

• You had a previous cesarean section with a vertical incision of the uterus.
• There is a chance that the placenta will separate from the uterus early (placental abruption).
• You have more than one baby in the uterus.
• The placenta is low in the uterus (placenta previa).
• You have premature rupture of your membranes (your bag of waters has broken).

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