From the Guidelines
A nonstress test (NST) is a crucial non-invasive prenatal test used to monitor fetal well-being, and it is typically recommended for high-risk pregnancies starting at 28-32 weeks gestation and continuing weekly or twice weekly until delivery, as indicated by the most recent study 1. The indications for an NST include maternal conditions like diabetes, hypertension, or decreased fetal movement, as well as pregnancy complications such as intrauterine growth restriction, post-term pregnancy, or previous stillbirth.
- The test is based on the physiological principle that a healthy fetus with adequate oxygen supply will show heart rate accelerations in response to movement, indicating intact central nervous system function and good oxygenation.
- During the test, which takes about 20-40 minutes, the mother reclines while two belts are placed on her abdomen—one to monitor fetal heart rate and another to detect contractions.
- A reactive (normal) NST shows at least two accelerations of fetal heart rate of 15 beats per minute above baseline, lasting 15 seconds within a 20-minute period, as defined in the study 1.
- If these criteria aren't met, the test may be extended or additional testing like a biophysical profile might be ordered, as suggested by the study 1. The NST is generally reliable, but it should be interpreted in the context of the overall clinical picture, taking into account other factors such as fetal movement, uterine activity, and the results of other prenatal tests, as recommended by the study 1.
- The use of NST in conjunction with other tests, such as the biophysical profile, can provide a more comprehensive assessment of fetal well-being, as indicated by the study 1.
- The American College of Radiology recommends the use of NST as part of the assessment of fetal well-being, particularly in high-risk pregnancies, as stated in the study 1.
From the Research
Nonstress Test in Pregnancy
The nonstress test is a method used to assess fetal well-being during pregnancy, particularly in the second and third trimesters 2. It serves as a surrogate measure of the developing fetal autonomic nervous system and is more specific than sensitive, making it a better indicator of fetal health than fetal illness.
Purpose and Interpretation
The nonstress test is used to:
- Document fetal well-being 2
- Identify high-risk maternal conditions 2
- Assess fetal health in high-risk pregnancies 3, 4 The test is read as reactive or nonreactive and may be repeated at intervals as a screen for high-risk maternal conditions 2.
Comparison with Other Testing Methods
The nonstress test is one of several methods used for antenatal fetal assessment, including:
- Biophysical profile 3, 5, 4
- Contraction stress test 3, 5, 4
- Vibroacoustic stimulation 5
- Amniotic fluid volume assessment 5 The biophysical profile is considered more accurate in identifying hypoxic fetuses than any other single method 3.
Clinical Applications
The nonstress test is used in various clinical settings, including: