A Non stress test (NST) is a specialized test done during pregnancy that measures the baby’s heart rate in response to the baby’s movements. It does not produce any stress on the baby, hence the name. This test is only done if your obstetrician feels your pregnancy is at a higher risk than normal for certain medical conditions.
The Non stress test procedure
The pregnancy stress test is done only after 28 weeks of pregnancy, before that the test is unreliable. The process takes about 20-30 minutes. You will be asked to lie down and two belts will be placed on your abdomen, one to measure the fetal heart rate while the other to measure any contractions that might occur. You will be asked to notify whenever your baby moves so that your obstetrician can check if the baby’s heart rate increases upon movement.
Non stress test interpretation
The Non stress test results back as either - Reactive or Nonreactive.
Reactive (Normal): The result is considered reactive if there are two or more increases in fetal heart rate within a 20 minute period. This indicates that the blood flow and oxygen supply to your baby is adequate.
Nonreactive: A nonreactive result means enough increments were not detected in a 40 minute period. It could mean several things like:
- Your baby was asleep during the test. In that case, the test may last over 40 minutes and your baby will be stimulated to move manually or by using a device to make a sound over the abdomen.
- You are taking certain medications or you smoke.
- There is not enough oxygen supply to the baby.
In case of a nonreactive test, your obstetrician will recommend a second test, or delivery if you are full-term. If you are not full term, further tests might be recommended which include
Biophysical profile: This test combines Non stress test with an ultrasound to check for your baby’s heart rate, movements, muscle tone and amount of amniotic fluid.
Contraction stress test: This test evaluates how your baby’s heart responds to uterine contractions. It is performed in the same way as a Non stress test. To make your uterus contract mildly, you will be asked to rub your nipples or you may be given a drug called oxytocin.
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