Question
1. Reactive NST indicates a healthy fetus
2. NST is an observed association of fetal breathing with fetal movements
3. NST has a low false negative rate (< 1%) but high false positive rate (>50%)
4. Testing should be started at 20 weeks
Which of the statement(s) given above is/are correct?
| A. |
1 and 3 |
| B. |
2 only |
| C. |
3 only |
| D. |
1 and 4 |
|
Correct Answer » A Explanation |
 |
linical Reasoning for the Correct Answer:
– Statement 1:
– A reactive NST (presence of accelerations with fetal movement in a specified time window) ⇒ usually indicates a healthy fetus, sufficient oxygenation, and absence of significant acidosis.
– Statement 3:
– NST is highly sensitive (low false-negative rate
– Has a high false positive rate (>50%)—non-reactive results are often not due to pathology (e.g., fetal sleep, CNS depressant drugs), hence low specificity.
Why Option B (2 only) is Incorrect:
– Statement 2:
– NST does not evaluate fetal breathing movements with movements.
– NST assesses acceleration of fetal heart rate with fetal movements, not specifically breathing.
– Fetal breathing is addressed in a biophysical profile (BPP).
Why Option C (3 only) is Incorrect:
– Statement 3 is correct, but excluding the clearly correct statement 1 makes this answer incomplete.
Why Option D (1 and 4) is Incorrect:
– Statement 4:
– NST is not started at 20 weeks (fetal neurological maturity insufficient for reliable results at this stage).
– Usually performed from 32–34 weeks onward or in high-risk pregnancies.
– Before that, fetal cardiac response is unreliable.
Why Each Statement is Correct or Incorrect:
– Statement 1:
– Correct: Reactive pattern = reassuring for fetal health.
– Statement 2:
– Incorrect: NST ≠ association of breathing and movement; it tests heart rate response to movement.
– Statement 3:
– Correct: Sensitivity high (low false-negative); specificity low (high false-positive).
– Statement 4:
– Incorrect: Initiation before 28–32 weeks = unreliable.
Non-Stress Test (NST):
– Purpose:
– Assesses fetal well-being by monitoring fetal heart rate (FHR) response to movements.
– Key Components:
– Evaluates FHR accelerations in response to perceived fetal movement.
– Does not evaluate breathing or muscle tone (feature of biophysical profile).
– Interpretation:
– Reactive NST: ≥ 2 accelerations (↑ in FHR by ≥ 15 bpm lasting ≥ 15 seconds) within 20 minutes (after 32 weeks).
– Non-reactive NST: Fails above criteria—needs further evaluation (BPP, contraction stress test).
– Sensitivity & Specificity:
– High sensitivity (false-negative rate – Low specificity (false-positive rate >50%): many non-reactive results are false alarms.
– Timing:
– Not reliable before 28–32 weeks; most often done ≥32 weeks or earlier in certain high-risk pregnancies.
– Differentials:
– NST vs. BPP: BPP includes breathing, tone, amniotic fluid, plus NST.
– NST vs. Contraction Stress Test: Contraction stress assesses FHR response to contractions; NST is at rest.
– Reactive NST rules out acute fetal hypoxia (high NPV).
– Non-reactive NST is not always ominous (low PPV).
– Test should be performed at ≥32–34 weeks, not at 20 weeks.
