What is the name of the fetal stress test used to assess fetal well‑being during pregnancy?

Fetal Stress Testing in Pregnancy

Direct Answer

The primary fetal stress test used during pregnancy is the Non-Stress Test (NST), also called cardiotocography, which has replaced the oxytocin stress test as the preferred method for heart rate-based fetal assessment because it is non-invasive, less time-consuming, does not require intravenous access, and is easier to interpret and repeat. 1

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Primary Testing Modalities

Non-Stress Test (NST)

• The NST is the first-line screening tool for assessing fetal well-being in high-risk pregnancies, evaluating fetal heart rate patterns in response to fetal movement 1, 2
• A reactive (normal) NST is defined as ≥2 fetal heart rate accelerations of 15 beats/minute above baseline, lasting 15 seconds, occurring within 20 minutes of observation 1, 3
• A reactive NST provides exceptional reassurance, with stillbirth occurring in only 0.8 per 1,000 cases (0.08%) within one week of a normal test 1
• The test typically takes 20-40 minutes to complete, accounting for fetal sleep cycles that can temporarily decrease variability 3

Modified Biophysical Profile (Modified BPP)

• The modified BPP combines NST with amniotic fluid volume assessment, creating the recommended approach for most high-risk pregnancies rather than proceeding directly to full BPP 1
• This combines acute assessment (NST for immediate oxygenation) with chronic assessment (amniotic fluid for placental function over the preceding week) 1
• Maximum vertical pocket (MVP) ≥2 cm is considered normal amniotic fluid throughout gestation 1

Full Biophysical Profile (BPP)

• Reserved for situations when NST is non-reactive or amniotic fluid is abnormal, the full BPP evaluates five parameters: fetal breathing movements, discrete body movements, fetal tone, amniotic fluid volume, and NST 1
• Each parameter scores 0 or 2 points, with scores of 8-10 considered normal and highly reassuring 1
• The ultrasound examination continues until all four ultrasound components meet criteria or 30 minutes have elapsed, whichever comes first 1

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Clinical Application Algorithm

When to Initiate Testing

• Antenatal fetal surveillance should be reserved exclusively for high-risk pregnancies, as routine testing in low-risk pregnancies causes iatrogenic prematurity from false-positive results 1
• High-risk conditions requiring surveillance include: advanced maternal age, obesity, hypertensive disorders, diabetes, chronic renal disease, thyroid disorders, thrombophilia, history of unexplained stillbirth, intrauterine growth restriction, decreased fetal movement, multiple gestations, fetal arrhythmias, and amniotic fluid abnormalities 1
• Testing typically begins at 32-34 weeks' gestation in high-risk pregnancies, though timing must be individualized based on the specific indication and likelihood of neonatal survival 1

Testing Frequency

• Weekly or twice-weekly testing has become standard clinical practice in high-risk pregnancies, though the optimal interval lacks rigorous scientific evidence 1
• Daily or more frequent testing may be indicated in critical situations such as growth restriction or abnormal Doppler findings 1

Management Based on Results

If NST is Reactive:

• Continue surveillance at established intervals (weekly or twice-weekly) 1
• No further testing needed unless new clinical concerns arise 1

If NST is Non-Reactive:

• Proceed immediately to modified BPP or full BPP 1, 4
• Do not use non-reactive NST alone for delivery decisions; always perform additional testing 1
• If BPP score is 8-10, continue surveillance with increased frequency (twice weekly) for preterm pregnancies 1
• If BPP score is ≤6, deliver immediately regardless of gestational age 1
• At term gestation (≥37 weeks), abnormal antenatal test results including non-reactive NST warrant delivery 4

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Additional Surveillance Tools

Umbilical Artery Doppler

• Reserved exclusively for suspected intrauterine growth restriction (IUGR) in high-risk pregnancies, reducing perinatal deaths by 29% 1
• Absent or reversed end-diastolic flow indicates severe placental compromise and requires urgent management 1
• Delivery recommended at ≥34 weeks for absent end-diastolic flow and ≥32 weeks for reversed end-diastolic flow, after corticosteroids 1

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Critical Limitations and Pitfalls

What Tests Cannot Predict

• No antenatal test, regardless of type or frequency, can predict stillbirth related to acute events such as placental abruption or cord accidents 1, 5
• False negatives, though uncommon, are typically associated with cord accidents, congenital anomalies, and abruptio placentae 5

Common Interpretation Errors

• Fetal sleep cycles lasting 20-40 minutes can cause temporary decreased variability without indicating fetal compromise 3
• Maternal medications (analgesics, anesthetics, barbiturates, magnesium sulfate) can decrease fetal heart rate variability without representing true fetal distress 3
• Maternal fever, infection, hyperthyroidism, or medications can cause fetal tachycardia (>160 bpm) 3

Avoiding Harm

• Never perform routine NST screening in low-risk pregnancies, as this increases iatrogenic prematurity without improving outcomes 1
• Do not rely on Doppler ultrasound in low-risk populations, as it provides no benefit and may cause harm from false-positive results 1
• Always assess amniotic fluid when performing NST, as oligohydramnios is an independent risk factor for stillbirth 1