What is the purpose and procedure of a Non-Stress Test (NST) in Obstetrics and Gynecology (OBG) for a pregnant woman, particularly those in their third trimester with high-risk pregnancies or concerns about fetal health?

Non-Stress Test (NST) in Obstetrics

Purpose of NST

The Non-Stress Test is a non-invasive fetal heart rate monitoring procedure that assesses immediate fetal oxygenation and acid-base balance in high-risk pregnancies, serving as the primary screening tool for fetal well-being because it is simpler, faster, and easier to interpret than alternative methods like the oxytocin stress test. 1

• NST evaluates acute fetal well-being by detecting fetal heart rate accelerations in response to fetal movement, which indicates intact central nervous system function and adequate oxygenation 1, 2
• The test has replaced the contraction stress test as the preferred heart rate-based assessment method because it requires no intravenous access, is less time-consuming, and can be easily repeated 1
• A reactive (normal) NST has an exceptionally high negative predictive value >99.9%, with stillbirth risk within 1 week of only 0.8 per 1,000 cases (0.08%) 1

Specific Indications for NST

Reserve NST exclusively for high-risk pregnancies, as routine testing in low-risk women causes iatrogenic prematurity from false-positive results. 1, 2

High-risk conditions requiring NST surveillance include: 1

• Advanced maternal age and obesity
• Hypertensive disorders and diabetes
• Chronic renal disease and thyroid disorders
• Thrombophilia and connective tissue disease
• History of unexplained stillbirth
• Intrauterine growth restriction (IUGR)
• Decreased fetal movement
• Multiple gestations
• Fetal arrhythmias
• Amniotic fluid abnormalities (oligohydramnios or polyhydramnios)

NST Procedure and Technical Performance

Monitor fetal heart rate continuously for 20 minutes initially, extending to 40 minutes if needed before declaring the test nonreactive. 2

Equipment Setup

• Use a cardiotocograph device that records fetal heart rate via continuous-wave Doppler ultrasound along with uterine activity 2
• Position the external transducer on the maternal abdomen to obtain optimal fetal heart rate signal 2

Monitoring Protocol

• Begin with 20 minutes of continuous observation 2
• If nonreactive after 20 minutes, extend monitoring to 40 minutes before declaring it nonreactive 2
• The extended timeframe accounts for fetal sleep cycles, which typically last 20-40 minutes and are the most common cause of nonreactive NST 2

Interpretation Criteria

At ≥32 weeks gestation, a reactive NST 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. 2

• Before 32 weeks gestation, criteria are modified to two or more accelerations of at least 10 beats/minute above baseline, lasting at least 10 seconds, reflecting developmental differences in fetal autonomic function 2
• A nonreactive NST is defined as one or no acceleration meeting the above criteria after 40 minutes of observation 2

Timing and Frequency of Testing

Initiate antepartum fetal surveillance at 32-34 weeks gestation for most high-risk pregnancies, balancing neonatal survival likelihood with the ability to detect fetal compromise. 1, 3, 2

• When fetal growth restriction is identified before 32 weeks, begin testing after the age of viability (typically 24 weeks) 3
• Weekly or twice-weekly testing has become standard clinical practice, though the optimal interval lacks rigorous scientific evidence 1, 2
• In critical situations with severe early-onset fetal growth restriction or abnormal Doppler findings, daily testing may be necessary 1, 3

Management Based on NST Results

Reactive NST

A reactive NST is highly reassuring and requires no immediate intervention, with continued surveillance at the established frequency. 1

• Stillbirth within 1 week of a reactive NST occurs in only 0.8 per 1,000 cases 1
• Continue weekly or twice-weekly surveillance as clinically indicated 1, 2

Nonreactive NST

Proceed immediately to modified biophysical profile (NST + amniotic fluid assessment) or full biophysical profile after a nonreactive NST—never use nonreactive NST alone for delivery decisions. 1

Management Algorithm for Nonreactive NST:

1. Perform modified biophysical profile or full biophysical profile immediately 1
2. If biophysical profile score is 8-10: Continue surveillance with increased frequency (twice weekly) for preterm pregnancies 1
3. If biophysical profile score is ≤6: Deliver immediately, regardless of gestational age 1
4. If oligohydramnios is detected: Deliver at term 1
5. If abnormal umbilical artery Doppler findings: Deliver immediately 1

Modified Biophysical Profile

Combine 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

• Amniotic fluid volume reflects chronic placental function over the preceding week 1
• A maximum vertical pocket (MVP) ≥2 cm is considered normal throughout gestation 1
• Amniotic fluid index (AFI) ≥5 cm at ≥37 weeks or ≥8 cm at <37 weeks is considered normal 1
• Oligohydramnios may indicate uteroplacental insufficiency and is an independent risk factor for stillbirth 1

Critical Limitations and Pitfalls

No antenatal test, including NST, can predict stillbirth related to acute events such as placental abruption or cord accidents, regardless of test frequency or type. 1, 3, 2

Common Causes of False Negative NST:

• Cord accidents (most common) 4
• Placental abruption 4
• Congenital anomalies 4

Pitfalls to Avoid:

• Never declare NST nonreactive before 40 minutes of monitoring—fetal sleep cycles are the most common cause of nonreactive NST 2
• Never ignore amniotic fluid assessment—oligohydramnios is an independent risk factor for stillbirth 1
• Never use NST in low-risk pregnancies—this causes iatrogenic prematurity from false-positive results 1, 2
• Never rely on NST alone for delivery decisions when nonreactive—always perform additional testing with biophysical profile 1
• Consider fetal acoustic stimulation to awaken the fetus and reduce unnecessary interventions 2

Role in Multiple Gestations

NST surveillance in multiple gestations is as reliable as in singleton pregnancies when complicated by abnormal fluid volumes, pregnancy-induced hypertension, fetal anomalies, growth abnormalities, or monoamnionicity. 5

• Umbilical artery Doppler evaluation is particularly helpful in monochorionic twins and when growth delay is suspected 5
• Abnormal Doppler findings can be detected as early as 16-20 weeks gestation and are associated with increased risk of adverse outcome and fetal demise 5