What is the preferred initial assessment between Non-Stress Test (NST) and Contraction Stress Test (CST) for fetal well-being?

NST is the Preferred Initial Assessment for Fetal Well-Being

The Non-Stress Test (NST) should be used as the initial assessment method for fetal well-being in high-risk pregnancies, with the Contraction Stress Test (CST) reserved only for cases where the NST is non-reactive. 1

Rationale for NST as First-Line Testing

The NST has replaced the oxytocin stress test (CST) as the preferred initial method for heart rate-based fetal assessment because it offers multiple practical advantages 1:

• Non-invasive nature: No intravenous access or medication administration required 1
• Time efficiency: Takes only 20 minutes compared to the more time-consuming CST 2
• Ease of interpretation: Simpler to perform and repeat as needed 1
• Excellent negative predictive value: A reactive NST is highly reassuring, with false negatives (stillbirth within 1 week) being uncommon 3

Clinical Performance and Reliability

The NST demonstrates strong clinical efficacy as a screening tool:

• High reactivity rate: Approximately 85% of NSTs in high-risk pregnancies are reactive (normal) 4
• Strong correlation with outcomes: Reactive NSTs correlate with negative CSTs in 99.4% of cases 5
• Adequate screening: A precisely defined reactive NST (≥2 fetal heart rate accelerations in 20 minutes) is an adequate screening tool for high-risk pregnancies 5

When to Proceed to CST

The CST should be reserved as a secondary test in specific circumstances 1, 5:

• Non-reactive NST: When the NST fails to show adequate fetal heart rate accelerations after 20 minutes
• Additional concerning findings: When oligohydramnios or other abnormalities are detected alongside a non-reactive NST 1
• Poor correlation caveat: Only 24.8% of non-reactive NSTs correlate with positive CSTs, meaning most non-reactive NSTs will have negative CSTs 5

Important Clinical Context

No single antenatal test has been shown to be superior; all have high negative predictive values 3. The choice of NST as initial testing is based on practical considerations rather than superior predictive ability:

• Both NST and CST are highly significant predictors of morbidity (P < 0.001) 5
• The CST may be a slightly better predictor of morbidity than NST, but the difference does not justify its use as first-line testing given the practical disadvantages 5
• Fetuses with both a non-reactive NST and a negative sequential CST are at no increased risk for morbidity 5

Critical Limitations to Recognize

Regardless of which test is used, no antenatal testing method can predict stillbirth related to acute events such as placental abruption or cord accidents 3, 1. False negative NSTs, though uncommon (0.026% within 7 days), are typically associated with these unpredictable acute events 6:

• Cord accidents (most common cause of false negatives) 6
• Abruptio placentae 6
• Congenital anomalies 6

Practical Algorithm for Initial Assessment

1. Perform NST as initial test in all high-risk pregnancies requiring fetal surveillance 1
2. If reactive: Reassuring; repeat at appropriate intervals (weekly or twice-weekly based on risk factors) 3
3. If non-reactive: Proceed to either full biophysical profile (BPP) or CST for further evaluation 1, 5
4. Consider modified BPP: NST plus amniotic fluid assessment is often preferred over proceeding directly to CST 1