A nonstress test (NST) measures the fetal heart rate in response to the fetus's movements. The heart rate of a healthy fetus should increase when the fetus moves. The NST is usually performed in the last trimester of pregnancy.
How is a nonstress test performed?
The actual procedure for a NST may vary, but, generally, the procedure is as follows:
- The test is often performed in a special prenatal testing area of the hospital. It may also be performed in your doctor's office. You will lay down, and a belt with a transducer attached will be placed around your abdomen and positioned over your baby’s heartbeat. The transducer is called an external fetal heart rate monitor.
- Your baby’s heart rate will be recorded on the monitor and on a paper printout.
- You will push a button on the monitor each time you feel your baby move.
- The test will likely last about 20 to 40 minutes, depending on how active your baby is.
Sometimes, the test may be scheduled during your baby’s sleep cycle, so there won’t be much fetal movement. If this happens, the person performing the test may use a special acoustic (sound) device to awaken your baby. The acoustic device will be placed against your abdomen and will make a noise like a buzzer. The noise is not harmful to your baby, but it may help a sleepy fetus become more active. You may also be asked to eat or drink something to wake your baby.
Test results of the NST
Test results of the NST may be:
- Reactive (normal), which means they detected two or more fetal heart rate increases during the testing period (usually 20 minutes).
- Nonreactive, which means there was no change in the fetal heart rate when your baby moved. This may indicate a problem that requires further testing.
A nonreactive NST does not always mean there is a problem with your baby. He or she may simply be asleep. You could also get a nonreactive result because of fetal immaturity. It is common for preterm fetuses, especially those before 28 weeks, to have nonreactive nonstress tests. Additional prenatal testing may be necessary to rule out certain conditions.