A nonstress test (NST) is a test done during the last three to four months of pregnancy in certain high-risk situations, usually to evaluate the fetus' health. The NST is an "external" test, which means no invasive procedures are necessary to perform the test. It can be done in the doctor's office, a hospital, or a fetal-evaluation unit.
An NST normally takes 20-60 minutes. The NST is called a "non-stress" test because it evaluates the fetus without causing it any stress. A stress test, also known as a "contraction stress test," involves hospital admission and induction of contractions to determine fetal health. The NST is usually done while you lie down or sit in a comfortable chair.
Reasons for doing a nonstress test include certain high-risk conditions including:
- Decreased fetal movements
- Going beyond the due date
- High blood pressure
- Changes in baby's growth rate
While you are lying on your side on a bed or stretcher, two small monitors are placed on your abdomen and held in place using elastic belts. One monitor checks for contractions; the other records the fetal heartbeat. The two monitors are attached by cables to a larger machine, the electronic fetal monitor. You're given a pushbutton and asked to push it every time you feel the fetus move. Your button pushes are recorded and then compared with the fetal heartrate tracing, the number of fetal movements, and the response of the fetus' heart to the movements.
The test results may be either reactive or nonreactive:
- A reactive NST is one that shows several fetal movements accompanied by fetal heart rate accelerations of at least 15 beats per minute lasting at least 15 seconds during a 15- to 20-minute period. A reactive NST usually means that the fetus is healthy and is getting enough nutrition.
- A nonreactive NST shows no increase in the fetal heart rate (no accelerations).
A nonreactive NST doesn't necessarily mean there's a problem. In fact, in a large proportion of cases, the fetus is perfectly healthy despite the nonreactive NST. A nonreactive result usually indicates that further testing should be done, such as a biophysical profile or a contraction stress test.