Contraction Stress Test
A contraction stress test checks to see if your unborn baby (fetus) will stay healthy during the reduced oxygen levels that normally occur during contractions when you are in labor. This test includes external fetal heart monitoring. The test is done when you are 34 or more weeks pregnant.
During a uterine contraction, the blood and oxygen supply to your baby drops for a short time. This is not a problem for most babies. But the heart rate of some babies gets slower. This change in heart rate can be seen on the external fetal monitoring device.
For a contraction stress test, the hormone oxytocin is given to you in a vein (intravenously, or IV) to cause labor contractions. You may also massage your nipples. This tells your body to release oxytocin. If your baby's heart rate slows down (decelerates) in a certain pattern after a contraction instead of speeding up (accelerating), your baby may have problems with the stress of normal labor.
A contraction stress test is usually done if you have an abnormal nonstress test or biophysical profile. A biophysical profile uses ultrasound during a nonstress test to measure a series of physical characteristics of your baby. You may have more than one contraction stress test during your pregnancy.
Some doctors may do a biophysical profile or a Doppler ultrasound test instead of a contraction stress test.
Why It Is Done
A contraction stress test is done to check:
- If your baby will stay healthy during the reduced oxygen levels that normally occur during contractions during labor.
- If the placenta is healthy and can support your baby.
A contraction stress test may be done when results from a nonstress test or a biophysical profile are not in the normal range.
How To Prepare
You may be asked to not eat or drink for 4 to 8 hours before the test. Empty your bladder before the test.
If you smoke, stop for 2 hours before the test, because smoking can lower your baby's activity and heart rate.
You will be asked to sign a consent form that says you understand the risks of the test and agree to have it done.
Talk to your doctor about any concerns you have regarding the need for the test, its risks, how it will be done, or what the results will mean. To help you understand the importance of this test, fill out the medical test information form(What is a PDF document?).
How It Is Done
A contraction stress test may be done in your doctor's office or hospital by a family medicine doctor or an obstetrician and a trained laboratory technician or nurse. You usually do not need to stay overnight.
During the test, you will lie on a bed with your back raised. You will be tilted a little to your left side so you will not have pressure on the blood vessels in your belly. Two belts with sensors will be placed around your belly. One belt holds the sensor that records your baby's heart rate; the other sensor measures your uterine contractions. Gel may be used on your skin with the heart rate sensors. The sensors are hooked to a recording unit. The heart rate monitor may be moved if your baby changes position. Your baby's heart rate and your contractions are recorded for 10 minutes. Your blood pressure and other vital signs are also recorded.
The hormone oxytocin is given in a low dose and increased until you have three contractions within 10 minutes, each one lasting longer than 45 seconds. Or you may be asked to massage one of your nipples by hand to start contractions. If you don't have a second contraction within 2 minutes of the first, you will massage your nipple again. If contractions do not occur within 15 minutes, you will stimulate both nipples.
After the test, you will be watched until your contractions go away or slow down to what they were before the test. A contraction stress test may take 2 hours.
How It Feels
You may need to lie on your left side for the test. This position may be uncomfortable or painful when you are having labor contractions. The belts holding the sensors may be uncomfortable. Most women say this test is uncomfortable but not painful.
Fetal heart monitoring may indicate that your baby is having problems when your baby is actually healthy. Fetal heart monitoring can't detect every type of problem, such as a birth defect.
The risks from having oxytocin include:
- Causing labor to start earlier than your expected delivery date.
- Causing prolonged contractions that may cause problems with your baby. The contractions usually stop when the oxytocin is stopped. You may be given a medicine to stop the contractions. If in the very rare case that your contractions do not stop, your doctor may recommend delivery.
A contraction stress test checks to see if your unborn baby (fetus) will stay healthy during the reduced oxygen levels that normally occur during contractions when you are in labor.
Results of the test tell your baby's health for 1 week. The test may need to be done more than once during your pregnancy.
Normal test results are called negative.
Your baby's heart rate does not get slower (decelerate) and stay slow after the contraction (late decelerations). Note: There may be a few times during the test when your baby's heart rate decelerates, but it doesn't stay slow so it is not a problem.
If three contractions occur during a 10-minute period of nipple stimulation or oxytocin infusion and there are no late decelerations in your baby's heart rate, your baby is expected to be able to handle the stress of labor.
Abnormal test results are called positive.
Your baby's heart rate gets slower (decelerates) and stays slow after the contraction (late decelerations). This happens on more than half of the contractions.
Late decelerations may mean that your baby will have problems during normal labor.
What Affects the Test
Reasons you may not be able to have the test or why the results may not be helpful include:
- Past pregnancy problems, such as a cesarean section with a midline (vertical) incision, placenta previa, or placenta abruptio. The test also is not recommended if you are pregnant with more than one baby, are likely to have premature rupture of the membranes (PROM), have an incompetent cervix, or have been given magnesium sulfate during your pregnancy.
- A uterine surgery in the past. Strong contractions may cause the uterus to rupture.
- If you smoke or use cocaine.
- Movements of your baby during the test. It may be hard for the sensors to record your baby's heart rate or contractions.
- Being very overweight.
What To Think About
- A contraction stress test may show decelerations when your baby is not actually having problems. This is called a false-positive result.
- Some doctors use oxytocin instead of nipple stimulation because nipple stimulation can cause long, uncontrolled contractions.
- Fischbach FT, Dunning MB III, eds. (2009). Manual of Laboratory and Diagnostic Tests, 8th ed. Philadelphia: Lippincott Williams and Wilkins.
Other Works Consulted
- Fischbach FT, Dunning MB III, eds. (2004). Manual of Laboratory and Diagnostic Tests, 7th ed. Philadelphia: Lippincott Williams and Wilkins.
- Handbook of Diagnostic Tests (2003). 3rd ed. Philadelphia: Lippincott Williams and Wilkins.
- Pagana KD, Pagana TJ (2010). Mosby’s Manual of Diagnostic and Laboratory Tests, 4th ed. St. Louis: Mosby.
|Primary Medical Reviewer||Sarah Marshall, MD - Family Medicine|
|Specialist Medical Reviewer||William Gilbert, MD - Maternal and Fetal Medicine|
|Last Revised||June 18, 2012|
Last Revised: June 18, 2012
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