Nonstress Test (NST)
The Nonstress test (NST) is a prenatal test used to check on the well-being of a baby, particularly in the third trimester of pregnancy. It measures the fetal heart rate in response to the baby's movements.
Purpose
The purpose of the NST is to:
- Assess fetal well-being and detect any potential problems
- Monitor the fetal heart rate and its response to movement
- Identify babies who may be at risk for complications, such as growth restriction or placental insufficiency
Procedure
The NST procedure typically involves:
- A pregnant woman lying on her back or side with a fetal monitor strapped to her abdomen
- A sensor that detects the fetal heart rate and movement
- A printout of the fetal heart rate tracing, which is analyzed for accelerations (increases in heart rate) and decelerations (decreases in heart rate)
- The test usually lasts between 20-40 minutes, although it may take longer if the baby is not active
Interpretation of Results
The results of the NST are interpreted as follows:
- Reactive: The test is considered reactive if there are two or more accelerations of at least 15 beats per minute (bpm) within a 20-minute period. This suggests that the baby is healthy and active.
- Nonreactive: If there are fewer than two accelerations, the test is considered nonreactive. This may indicate that the baby is sleeping or under stress, but it can also be a sign of potential problems.
Risks and Limitations
The NST is a non-invasive and safe procedure, but it has some limitations:
- False reassurance: A reactive test does not guarantee that the baby is healthy or that there are no underlying problems.
- False alarms: A nonreactive test may lead to further testing or interventions that may not be necessary.
Follow-up Testing
If the NST is nonreactive, follow-up testing may include:
- Biophysical profile (BPP): An ultrasound test that evaluates fetal movement, tone, breathing, and other parameters.
- Contraction stress test (CST): A test that measures the fetal heart rate in response to uterine contractions.
Article last updated on: 15th June 2025.
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