Non-Stress Test vs Cardiotocography: Understanding the Terminology
A non-stress test (NST) and cardiotocography (CTG) are the same test—they both refer to continuous electronic monitoring of fetal heart rate and uterine activity using a cardiotocograph device. The terms are used interchangeably, with "NST" being more common in North American practice and "CTG" more prevalent in European and British literature 1, 2.
Technical Definition and Equipment
Both NST and CTG utilize a cardiotocograph device that records fetal heart rate via continuous-wave Doppler ultrasound along with uterine activity on a composite tracing 3, 2.
The external transducer is positioned on the maternal abdomen to obtain optimal fetal heart rate signal, creating a continuous record of both fetal heart rate and uterine contractions 3, 2.
Clinical Applications: When Each Term Is Used
During Pregnancy (Antepartum)
The term "non-stress test" (NST) is preferred when referring to antepartum fetal surveillance in high-risk pregnancies, where the test assesses fetal well-being without inducing uterine contractions 1, 3.
The American College of Radiology recommends NST as the primary heart rate-based assessment method because it is non-invasive, less time-consuming than stress tests, requires no intravenous access, and is easier to interpret and repeat 1.
NST should be initiated at 32-34 weeks' gestation for most high-risk conditions, including advanced maternal age, hypertensive disorders, diabetes, intrauterine growth restriction, decreased fetal movement, and amniotic fluid abnormalities 1, 3.
During Labor (Intrapartum)
The term "cardiotocography" (CTG) is more commonly used during labor, where it provides continuous indication of fetal response to uterine contractions and interventions 2.
Continuous electronic fetal monitoring (CTG) is recommended during labor for cases with decreased fetal movement concerns or other high-risk features 1.
Test Performance and Interpretation
Standard Protocol
Monitor fetal heart rate for 20 minutes initially, extending to 40 minutes if needed before declaring the test non-reactive 3.
The extended timeframe accounts for fetal sleep cycles, which typically last 20-40 minutes and are the most common cause of non-reactive results 3.
Reactivity Criteria
At ≥32 weeks gestation: A reactive test requires two or more fetal heart rate accelerations of at least 15 beats/minute above baseline, lasting at least 15 seconds, within a 20-minute observation period 3.
Before 32 weeks gestation: Two or more accelerations of at least 10 beats/minute above baseline, lasting at least 10 seconds, reflecting developmental differences in fetal autonomic function 3.
Research confirms that 30 minutes is the optimal duration for NST/CTG to minimize false-positive results, with significantly more reactive patterns identified after 30 minutes compared to 20 minutes 4.
Clinical Significance and Limitations
Predictive Value
A reactive NST/CTG has exceptional reassurance value, with stillbirth risk within 1 week of only 0.8 per 1,000 cases (0.08%) 1.
The negative predictive value exceeds 99.9% for fetal well-being 1.
Critical Limitations
No antenatal test—whether called NST or CTG—can predict stillbirth related to acute events such as placental abruption or cord accidents, regardless of test frequency 1, 3.
Antenatal fetal surveillance should be reserved exclusively for high-risk pregnancies, as routine testing in low-risk women causes iatrogenic prematurity from false-positive results without improving outcomes 1, 3.
Integration with Other Testing
The American College of Radiology recommends combining NST with amniotic fluid assessment to create a "modified biophysical profile" that evaluates both acute (NST) and chronic (amniotic fluid) markers of fetal well-being 1.
If NST/CTG is non-reactive, proceed immediately to modified biophysical profile or full biophysical profile rather than using the non-reactive result alone for delivery decisions 1.
Fetal acoustic stimulation can shorten testing time and reduce false-positive results when performing NST/CTG 1.
Common Pitfalls to Avoid
Always extend monitoring to 40 minutes before declaring a test non-reactive, as fetal sleep cycles are the most common cause of apparent non-reactivity 3.
Do not use NST/CTG in low-risk pregnancies, as this increases unnecessary interventions without benefit 1, 3.
Consider maternal position during testing—sitting position or walking encourages faster fetal reactivity compared to reclining, particularly as pregnancy progresses 5.