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:
- The test is performed in our 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.
- If mom is having contractions, this is also recorded on the monitor and on paper printout.
- Testing usually lasts for 20 minutes.
Sometimes, the testing occurs during a fetal sleep cycle, when there is little fetal movement. Having
the mother eat or drink before testing will keep fetus awake and active during testing.
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.
