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Contraction Stress Test

What is a contraction stress test?

A contraction stress test uses fetal monitoring to check the health of an unborn baby. An external fetal monitor is attached with belts to the mother's abdomen (belly) to record the baby's heart rate during contractions. If you are not having contractions, you may be given the medicine oxytocin to cause contractions. During the 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. Normally, a healthy baby's heart rate does not change during contractions.

This test is also called an oxytocin challenge test.

When is it used?

A contraction stress test is usually done the 32nd week of pregnancy or later. If a pregnancy is high risk, the first test may be done at 26 to 28 weeks.

This test may be done if:

  • During a nonstress test the baby's heart rate did not rise enough during movements 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 do I prepare for this test?

  • Pregnant women should not smoke, but if do you smoke, do not smoke for at least 4 hours before the test. Smoking can decrease your baby's movements.
  • Ask your provider if you need to avoid eating for 4 to 8 hours before the test.

What happens during the test?

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 to measure the baby's heart rate. Your blood pressure, the baby's heart rate, and contractions of the uterus will be recorded for several minutes.

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. Your healthcare provider may give you a very small amount of oxytocin through a vein (IV) to start contractions if you are not having contractions that are strong and frequent enough. The oxytocin may be given until you have 3 contractions, each lasting 40 seconds, in 10 minutes.

How is the test interpreted?

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

  • If the baby's heart rate does not slow during the contractions, the result of the test is normal. (A normal result is called negative.) This is a reassuring result that suggests that the baby is healthy.
  • If the baby's heart rate slows during a contraction, the result of the test may be interpreted as abnormal and nonreassuring. (An abnormal result is called positive.) 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 slowing 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 the 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 if your pregnancy is high risk.

If the result of the stress test is positive and nonreassuring, 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 (C-section).

What are the benefits of this procedure?

The contraction stress test allows your healthcare provider to check the baby's response to contractions. If the baby is not doing well, steps may be taken to help the baby. If the monitoring shows a normal pattern, it is reassuring to the mother and her provider.

What are the risks of a contraction stress test?

The stress test could cause too many uterine contractions, especially if you are given oxytocin to stimulate the contractions. It could make you go into labor. For this reason the oxytocin is given slowly and carefully. It is stopped if it is causing too many contractions.

Your healthcare provider may not use this test if:

  • You are at risk of going into premature labor.
  • You had a previous C-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 1 baby in the uterus.
  • The placenta is low in the uterus (placenta previa).
  • You have early rupture of your membranes (your bag of waters has broken).

Disclaimer: This content is reviewed periodically and is subject to change as new health information becomes available. The information provided is intended to be informative and educational and is not a replacement for professional medical evaluation, advice, diagnosis or treatment by a healthcare professional.

HIA File preg5312.htm Release 13/2010

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