What are the indications and differences between non-stress test (NST) and biophysical profile (BPP) for assessing fetal well-being?

Indications for Non-Stress Test (NST) and Biophysical Profile (BPP)

Both the Non-Stress Test (NST) and Biophysical Profile (BPP) are indicated for high-risk pregnancies to assess fetal well-being, with BPP providing more comprehensive evaluation through ultrasound assessment of multiple fetal parameters. 1

Non-Stress Test (NST)

Definition and Methodology

• NST is performed with a cardiotocograph that records fetal heart rate via continuous-wave Doppler ultrasound along with uterine activity
• Evaluates fetal heart rate accelerations in response to fetal movement
• Considered reactive if there are two or more fetal heart rate accelerations during 20 minutes of observation
• Considered nonreactive if after 40 minutes one or no acceleration is detected

Primary Indications for NST

1. High-risk pregnancies with increased risk of stillbirth:

   • Intrauterine growth restriction (IUGR)
   • Maternal hypertension
   • Diabetes mellitus
   • Advanced maternal age
   • Previous stillbirth
   • Post-term pregnancy
   • Decreased fetal movement
   • Preterm premature rupture of membranes
   • Abnormal maternal serum markers

2. Assessment of immediate fetal oxygenation and acid-base balance

Strengths of NST

• Provides information about immediate fetal status
• High negative predictive value (0.026% false negative rate) 2
• Relatively quick to perform
• Widely available in obstetric practice

Limitations of NST

• Higher false-positive rate compared to BPP
• Cannot detect acute events like cord accidents or placental abruption
• Limited in assessing chronic fetal compromise

Biophysical Profile (BPP)

Definition and Components

• Comprehensive assessment consisting of four ultrasound-based parameters plus optional NST
• Ultrasound components (each scored 0 or 2):
  1. Fetal breathing movements (≥1 episode for 30 seconds within 30 minutes)
  2. Discrete body/limb movements (≥3 movements)
  3. Fetal tone (≥1 episode of active extension with return to flexion)
  4. Amniotic fluid volume (≥1 pocket measuring 2×2 cm)
• NST (if included) adds 2 points if reactive
• Total score: 8/8 (without NST) or 10/10 (with NST)
• Score interpretation: 8-10 normal, 6 equivocal, ≤4 abnormal

Primary Indications for BPP

1. Same high-risk conditions as NST, but particularly valuable in:

   • IUGR with suspected uteroplacental insufficiency
   • Oligohydramnios or polyhydramnios
   • Abnormal NST results requiring further evaluation
   • Post-term pregnancy
   • Maternal vascular disease

2. Assessment of both immediate and chronic fetal well-being

Strengths of BPP

• More comprehensive assessment than NST alone
• Lower false-positive rate than NST
• Evaluates both acute and chronic aspects of fetal well-being
• Highly reassuring when normal (low false-negative rate)

Limitations of BPP

• More time-consuming than NST alone
• Requires specialized ultrasound equipment and trained personnel
• May still miss acute events occurring after testing
• Rare cases of false reassurance despite fetal compromise 3

Modified BPP (mBPP)

• Combines NST with ultrasound assessment of amniotic fluid volume
• Provides balance between comprehensive assessment and testing efficiency
• Particularly useful in IUGR and cases with suspected oligohydramnios 1

Comparative Analysis and Decision Algorithm

When to Choose NST:

• Initial screening in high-risk pregnancies
• When ultrasound resources are limited
• For frequent monitoring (e.g., twice weekly)
• When assessing immediate fetal status is the primary concern

When to Choose BPP:

• After a nonreactive NST
• In cases of severe IUGR
• When chronic placental insufficiency is suspected
• When comprehensive fetal assessment is needed
• In cases with suspected amniotic fluid abnormalities

Timing and Frequency

• Typically initiated at 32-34 weeks' gestation in high-risk pregnancies
• Earlier initiation may be warranted in severe cases
• Weekly or twice-weekly testing is standard clinical practice
• Frequency should increase with worsening maternal or fetal condition

Important Clinical Considerations

• Neither test can predict acute events like cord accidents or abruption
• A normal result for either test is highly reassuring but not absolute
• Abnormal results at term generally warrant delivery
• Antenatal testing is not recommended for low-risk pregnancies due to risk of iatrogenic prematurity from false positives
• In cases of IUGR, Doppler velocimetry of umbilical artery provides additional valuable information 1

Pitfalls to Avoid

1. Relying solely on NST when more comprehensive assessment is needed
2. Overconfidence in normal test results (both have rare false negatives)
3. Failure to recognize acute deterioration despite previous normal testing
4. Using antenatal testing in low-risk pregnancies (risk of iatrogenic prematurity)
5. Ignoring maternal perception of decreased fetal movement despite normal testing