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A nonstress test (NST) measures the fetal heart rate in response to the fetus' 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:
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:
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.