In a pregnancy ≥ 28 weeks with risk factors such as diabetes, chronic hypertension, pre‑eclampsia, intra‑uterine growth restriction, or decreased fetal movements, what is a non‑stress test, what defines a reactive result, and how should a non‑reactive test be managed?

Non-Stress Test in High-Risk Pregnancy ≥28 Weeks

What is a Non-Stress Test?

The non-stress test (NST) is a non-invasive cardiotocography procedure that evaluates immediate fetal oxygenation and acid-base balance by monitoring fetal heart rate patterns in response to fetal movement, serving as a surrogate measure of the developing fetal autonomic nervous system. 1, 2, 3

• The NST has replaced the oxytocin stress test as the preferred method for heart rate-based fetal assessment because it is non-invasive, less time-consuming, does not require intravenous access, and is easier to interpret and repeat 1
• The test provides real-time assessment of acute fetal well-being, distinguishing it from chronic markers like amniotic fluid volume 1
• NST is technically simple to perform and when reactive (negative), is a highly reliable predictor of fetal well-being for up to one week 2

Definition of a Reactive (Normal) NST

A reactive NST is defined as ≥2 fetal heart rate accelerations within a 20-minute observation period. 1

• A reactive NST is highly predictive of fetal well-being with a negative predictive value >99.9% 1
• Stillbirth risk within 1 week of a reactive NST is only 0.8 per 1,000 cases (0.08%) 1
• A reactive NST is predictive of good fetal health regardless of the length of observation time necessary to demonstrate reactivity, up to 120 minutes 4
• The test may be extended up to 40 minutes to allow for fetal sleep cycles before declaring it non-reactive 1

Management of a Non-Reactive NST

A non-reactive NST requires immediate further evaluation with either a full biophysical profile (BPP) or modified biophysical profile (mBPP) to determine if delivery is indicated; never use a non-reactive NST alone for delivery decisions. 1

Immediate Next Steps After Non-Reactive NST

• Proceed immediately to modified biophysical profile (NST + amniotic fluid assessment) or full biophysical profile after a non-reactive result 1
• The modified BPP combines acute assessment (NST for immediate oxygenation) with chronic assessment (amniotic fluid for placental function over the preceding week) 1

Management Algorithm Based on Further Testing

If BPP score is 8-10:

• Continue surveillance with increased frequency (twice weekly) for preterm pregnancies 1
• A BPP score of 8 or 10 has a perinatal mortality rate of only 1.0 per 1,000 and false-negative rate of 0.7 per 1,000 within one week 1

If BPP score is ≤6:

• Deliver immediately, regardless of gestational age 1

If oligohydramnios is detected (maximum vertical pocket <2 cm or AFI <5 cm at ≥37 weeks):

• Deliver at term 1
• Oligohydramnios may indicate uteroplacental insufficiency and is an independent risk factor for stillbirth 1

At term gestation (≥37 weeks) with non-reactive NST:

• Abnormal antenatal test results, including non-reactive NST with mBPP, are an indication for delivery at term 5, 6
• Induction of labor is reasonable if there are no other contraindications, but cesarean delivery may be warranted based on the complete clinical scenario 6

Additional Considerations for Specific High-Risk Conditions

For suspected intrauterine growth restriction (IUGR):

• Add umbilical artery Doppler assessment if IUGR is suspected, as absent or reversed end-diastolic flow is most predictive of adverse perinatal outcomes 5, 1
• Deliver at 33-34 weeks with absent end-diastolic flow or 30-32 weeks with reversed end-diastolic flow, after corticosteroids 7

For preeclampsia:

• Fetal monitoring should include initial assessment to confirm fetal well-being, with serial surveillance if growth restriction is present 5
• Deliver at 37 weeks or if maternal/fetal complications develop 5

Surveillance Protocol in High-Risk Pregnancies

• For most high-risk patients, antepartum fetal surveillance should be initiated at 32-34 weeks' gestation 5, 1
• Weekly or twice-weekly testing has become standard clinical practice, though the optimal interval lacks rigorous scientific evidence 5, 1
• NST surveillance should be reserved exclusively for high-risk pregnancies; routine testing in low-risk women causes iatrogenic prematurity from false-positive results without improving outcomes 5, 1

Critical Limitations and Caveats

No antenatal test, including NST, can predict stillbirth related to acute events such as placental abruption or cord accidents, regardless of test frequency or timing. 5, 1, 8

• False-negative NSTs (fetal death within 7 days of reactive test) occur in approximately 0.026% of cases and are typically associated with cord accidents, congenital anomalies, or abruptio placentae 8
• Close attention to the presence of fetal heart rate decelerations on the NST may help identify patients at risk for cord accidents 8
• The NST is more specific than sensitive, making it a better indicator of fetal health than fetal illness 3
• Never ignore amniotic fluid assessment, as oligohydramnios is an independent risk factor for stillbirth 1