What is a contraction stress test (CST)?

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The contraction stress test (CST) is used to determine if there are adequate fetal reserves prior to the onset of labor by evaluating the response of the fetal heart rate to spontaneous or induced uterine contractions. The goal of a CST is to induce uterine contractions and to evaluate the fetal response.

In a contraction stress test, a fetal heart rate monitor and uterine contraction monitor are placed around the maternal abdomen, and the fetal heart rate response to contractions is noted. The contractions may be spontaneous or induced through nipple stimulation and administration of oxytocin through an intravenous solution. In a healthy pregnancy, the placenta has the capacity to transport oxygen from maternal to fetal blood. If the placenta is compromised, less oxygen may be transported. A compromised placenta becomes evident when the uterus contracts and the vessels to the placenta are compressed, limiting blood flow and oxygen delivery. If oxygen passage across the placenta drops too much, the fetus responds with a decrease in heart rate. If the decrease in the fetal heart rate meets pre-determined criteria the CST is “Positive”. It there is no decrease in the fetal heart rate, the CST is negative. A negative CST is a reassuring finding whereas a positive CST is a sign that the fetus may not be receiving adequate oxygen and nutrients.

A CST is usually not performed if there are signs of premature labor, placenta previa, premature rupture of the membrane, incompetent cervix, or previous uterine incision or any other condition which makes labor contraindicated. One of the risks of a CST is that one initiated the uterine contraction will not cease and the labor will progress to birth of the fetus. In many settings a biophysical profile has replaced the CST, because it eliminated the concern related to onset of labor.

A contraction stress test is a prenatal screening late in pregnancy, prior to the onset of labor. It uses a fetal monitor to evaluate how the fetal heart reacts to uterine contractions. Certain fetal heart-tracing characteristics occur in both healthy and unhealthy fetuses. The contractions can be induced by a medication called oxytocin, administered intravenously. The stress created by these contractions may reveal that the fetus is receiving a marginal blood and oxygen supply.
Dr. Mehmet Oz, MD
Cardiology (Cardiovascular Disease)
This test is one way to tell if your baby is experiencing a stressful environment. After contractions are induced (either through oxytocin being given via an IV or through stimulation of the breasts), your doctor will look for a decrease in fetal heart rate occurring with more than 50 percent of the contractions they are inducing.

During contractions, the pressure of amniotic fluid increases, and because no blood is entering the uterus, there's less blood flowing into the space between the placenta and the uterus. So the capacity to maintain oxygen levels depends on the amount of working surface area available in the placenta.

If there are placental problems, you'll see the fetal heart rate slow down more than normal, as the oxygen delivered becomes dangerously low for your baby's heart. This is often used if no ultrasounds are available where you are getting care.
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Important: This content reflects information from various individuals and organizations and may offer alternative or opposing points of view. It should not be used for medical advice, diagnosis or treatment. As always, you should consult with your healthcare provider about your specific health needs.