The contraction stress test, or CST, is usually given during the third trimester.
The contraction stress test is used for assessing the health of the baby in high-risk cases, when a non-stress test has yielded suspicious results, or when a biophysical profile has yielded a low score. The CST measures the fetal heart rate during uterine contractions. Contractions briefly decrease the flow of blood to the placenta, so by monitoring fetal heart rate while they're happening, the CST can reveal if the baby will be able handle the stresses of labor.
Women who are at risk of premature labor will not be given the contraction stress test. Your practitioner may also avoid this test and opt for a biophysical profile if you are carrying more than one baby, if you've had a previous cesarean section, or if you are at risk for placental abruption or placenta previa.
During the CST, your health care provider measures contractions of your uterus by placing a strap with a pressure gauge around your abdomen. Your baby's heart rate is measured with an ultrasound transducer. The transducer is placed over the area of your abdomen where the baby's heart is. The monitor records changes in pressure from the uterus (contractions) and the baby's heart rate as two separate lines on graph paper, which are analyzed to determine the baby's well-being.
If contractions aren't happening on their own, they may be brought on by an intravenous dose of oxytocin (via a tiny needle inserted into your hand or arm) or by having you stimulate your nipples, which causes a natural releases of oxytocin, the hormone that signals your uterus to begin contracting. The test lasts until you have had three contractions of moderate strength within a ten-minute period.
Results are available immediately. A normal, or reactive, result means that your baby's heartrate hasn't slowed in response to contractions and so the baby probably doing fine in the uterus. An abnormal result occurs when a baby's heartrate does decrease following a contraction and indicates that the baby may be under stress. In this case, your practitioner will discuss further options with you and may advise that the baby be delivered early by labor induction or cesarean section.
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