Nst
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"nonstress test" OR "non-stress test"
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"Nst" wasn't clear, so I took it to mean NST, the nonstress test used in pregnancy. If you meant something else (another medical abbreviation or a typo), tell me and I'll redo it.
Nonstress test (NST)
What it is: A fetal heart rate (FHR) monitoring test. It checks for FHR accelerations that go with fetal movement. Accelerations indicate an intact fetal autonomic nervous system and adequate oxygenation. The NST is the usual first-line test of antenatal surveillance in high-risk pregnancies, even though no RCT supports it (Pfenninger and Fowler's Procedures for Primary Care, p. 1150).
Indications (generally from about 32 weeks, sometimes from 26 weeks):
- Suspected or confirmed intrauterine growth restriction
- Maternal or gestational diabetes
- Hypertensive disorders of pregnancy
- Post-term pregnancy
- Decreased fetal movement
- Maternal trauma
- Other risks, such as renal disease, multiple gestation, substance abuse or prior fetal demise
Frequency: Testing is weekly in low to moderate risk pregnancies (for example GDM, chronic hypertension or mild pre-eclampsia). It is twice weekly or more often for post-term pregnancy, type 1 diabetes, IUGR or gestational hypertension (Pfenninger and Fowler's Procedures for Primary Care; Textbook of Family Medicine 9e).
Contraindications: There are none. Stop the test if the mother goes into labor and the fetus tolerates it poorly.
Technique
- Put the mother in a semi-recumbent position. This avoids supine hypotension, which can cause a false nonreactive result.
- Monitor for a 30-minute baseline period.
- If the tracing is nonreactive, consider two more 30-minute periods. A low-risk fetus may simply be asleep, and sleep cycles can last up to 40 minutes.
- If fetal movement is insufficient, use vibroacoustic stimulation (VAS). ACOG allows 3 seconds, repeated up to three times. VAS does not reduce sensitivity or raise the rate of falsely reassuring tests. In one study it cut the average NST time from 24 to 15 minutes.
Interpretation
| Result | Criteria |
|---|---|
| Reactive (reassuring) | At least 2 accelerations of 15 bpm or more above baseline, each lasting 15 seconds or more and under 2 minutes, in a 20-minute period |
| Reactive, under 32 weeks | At least 2 accelerations of 10 bpm or more, each lasting 10 seconds or more (NICHD) |
| Nonreactive | No accelerations after fetal sleep has been excluded |
- ACOG also counts accelerations without perceived fetal movement as reactive.
- The NST is more specific than sensitive, so it is a better indicator of fetal health than of fetal illness. (Textbook of Family Medicine 9e, p. 500)
- A nonreactive NST does not mean the fetus is in jeopardy. It is a reason for further evaluation, usually a biophysical profile (BPP) or a contraction stress test (CST).
- Other causes of a nonreactive tracing: maternal narcotics or barbiturates, extreme prematurity, and fetal cardiac or CNS anomalies.
- False positives can lead to unnecessary interventions, including cesarean delivery.
Biophysical profile: The BPP has 5 components, each scored 0 or 2. A score of 8 to 10 is reassuring, 6 is equivocal, and 4 or less is ominous. A BPP is often used when the NST is equivocal.
Recent evidence check: A PubMed search of the last 2 years found one new systematic review and meta-analysis. It looked at virtual reality for maternal anxiety during NSTs (PMID 39847547). It does not change how the test is done or read.
I can go further on any of these: CST, BPP scoring, management of a nonreactive NST, or fetal heart rate tracing categories.
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