A nonstress test (NST) measures the fetal heart rate in response to the fetus's movements. Generally, the heart rate of a healthy fetus increases when the fetus moves. The NST is usually performed in the last trimester of pregnancy.
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, or in a doctor's office.
- The mother lies down and has a belt placed around her abdomen with a transducer positioned over the fetal heartbeat, called an external fetal heart rate monitor.
- The fetal heart rate is recorded on the monitor and on a paper printout.
- The mother pushes a button on the monitor each time she feels fetal movement.
- Testing usually lasts for 20 to 40 minutes.
Sometimes, the testing occurs during a fetal sleep cycle, when there is little fetal movement. A special acoustic (sound) device is sometimes used to awaken the fetus. It is placed against the mother's abdomen and makes a noise like a buzzer. This is not harmful to the fetus but may help a sleepy fetus become more active. Having the mother eat or drink may also awaken the fetus.
Test results of the NST may be:
- Reactive (normal). Two or more fetal heart rate increases in the testing period (usually 20 minutes).
- Nonreactive. There is no change in the fetal heart rate when the fetus moves. This may indicate a problem that requires further testing.
A nonreactive NST does not always mean there is a problem with the fetus. The fetus may simply be asleep. Or, it may be nonreactive 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.