3.2 Nonstress Test (NST), Contraction Stress Test (CST) & Biophysical Profile (BPP)
Key Takeaways
- A Nonstress Test (NST) is classified as Reactive at ≥32 0/7 weeks when ≥2 accelerations of ≥15 bpm above baseline lasting ≥15 seconds occur within a 20-minute window (15x15 rule); before 32 0/7 weeks, reactivity requires accelerations of ≥10 bpm lasting ≥10 seconds (10x10 rule).
- A Nonreactive NST lacks sufficient accelerations over a 40-minute period (accounting for normal fetal sleep cycles); initial nursing interventions include acoustic stimulation (Vibroacoustic Stimulation [VAS] for 3 seconds) and reflexing to a Biophysical Profile (BPP) or Contraction Stress Test (CST).
- A Contraction Stress Test (CST) requires ≥3 uterine contractions of ≥40 seconds duration in 10 minutes; a Negative CST exhibits no late or significant variable decelerations (highly reassuring, NPV >99%), whereas a Positive CST demonstrates late decelerations following ≥50% of contractions.
- CST is strictly contraindicated in conditions where myometrial contractions increase the risk of uterine rupture, hemorrhage, or preterm birth, including placenta previa, vasa previa, prior classical cesarean incision, extensive transmural myomectomy, cerclage, and PPROM.
- The standard Biophysical Profile (BPP) scores 5 parameters (Fetal Breathing Movements, Gross Body Movements, Fetal Tone, Amniotic Fluid Volume, and Reactive NST) with 2 points each (maximum 10/10); an abnormal Single Deepest Vertical Pocket (<2 cm) indicates oligohydramnios and mandates clinical action regardless of the total score.
Last updated: August 2026
The Nonstress Test (NST): Physiology, Technique, and Interpretation
The Nonstress Test (NST) is the most widely utilized initial method of antepartum fetal surveillance. The physiological foundation of the NST is that a healthy, well-oxygenated fetus with an intact, mature autonomic nervous system exhibits transient heart rate accelerations in response to fetal movement and uterine activity. These accelerations are mediated by sympathetic nervous system stimulation in response to somatic movement.
Clinical Technique & Patient Preparation
- Positioning: The patient is placed in a semi-Fowler's position with a lateral tilt (or left-lateral recumbent position) utilizing a wedge under the right hip to prevent aortocaval compression and supine hypotension syndrome, which causes decreased uteroplacental perfusion and artifactual fetal decelerations.
- Transducer Placement: An ultrasound transducer (measuring fetal heart rate) is placed over the fetal back, and a tocodynamometer (measuring uterine contractions and fetal movements) is secured over the uterine fundus.
- Testing Duration: A standard testing window is 20 minutes. If the tracing does not meet reactivity criteria within 20 minutes, monitoring must be extended to 40 minutes (and up to 60–80 minutes if needed) to account for normal fetal quiet sleep cycles (which typically last 20 to 40 minutes, rarely up to 75–90 minutes).
Gestational Age-Specific Criteria for Reactivity
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| GESTATIONAL AGE-BASED NST REACTIVITY CRITERIA |
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Gestational Age Range Reactivity Criteria Threshold (Within a 20- to 40-Minute Window)
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≥ 32 0/7 Weeks ★ "15 x 15 Rule": ≥ 2 qualifying accelerations peaking ≥ 15 bpm above baseline
and lasting ≥ 15 seconds from initial departure to return to baseline.
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< 32 0/7 Weeks ★ "10 x 10 Rule": ≥ 2 qualifying accelerations peaking ≥ 10 bpm above baseline
(24 0/7 to 31 6/7 weeks) and lasting ≥ 10 seconds from initial departure to return to baseline.
(Reflects developmental immaturity of the fetal sympathetic/parasympathetic axis).
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Nonreactive NST and Acoustic Stimulation Protocols
- Nonreactive NST: Defined by the absence of sufficient qualifying accelerations over a 40-minute testing period. While a nonreactive result can stem from fetal acidemia and hypoxemia, the false-positive rate of a nonreactive NST exceeds 75% to 80%, commonly caused by fetal sleep states, maternal medication administration (magnesium sulfate, opioids, sedatives, beta-blockers), fetal neurologic depression, or extreme prematurity.
- Vibroacoustic Stimulation (VAS): An artificial larynx device applied to the maternal abdomen over the fetal head for 1 to 3 seconds. VAS produces a combination of auditory and vibratory stimuli that startles the fetus, converting quiet sleep to active wakefulness. If no acceleration occurs, VAS may be repeated at 1-minute intervals up to 3 times (maximum total duration of 3 seconds per application). Inducing a qualifying acceleration following VAS confirms fetal well-being and confers the same negative predictive value as a spontaneous reactive NST.
- Variable Decelerations During NST: Non-repetitive, brief variable decelerations (<30 seconds) are common and usually benign. However, repetitive variable decelerations (≥3 in 20 minutes) or variable decelerations lasting ≥60 seconds indicate umbilical cord compression (often due to oligohydramnios) and mandate immediate ultrasound evaluation of amniotic fluid volume or reflex testing.
The Contraction Stress Test (CST) / Oxytocin Challenge Test (OCT)
The Contraction Stress Test (CST) evaluates the respiratory function of the placenta by assessing the fetal heart rate response to transient myometrial contractions. During a uterine contraction, intramyometrial pressure compresses maternal spiral arteries traversing the myometrium, temporarily decreasing or arresting blood flow into the intervillous space. A fetus with adequate placental respiratory reserve tolerates this transient hypoxemia without difficulty; a fetus with marginal uteroplacental oxygenation develops late decelerations.
Testing Methodology
- Prerequisite Contraction Pattern: The test requires achieving at least 3 uterine contractions of ≥40 seconds duration within a 10-minute window.
- Methods of Induction:
- Nipple Stimulation CST: Unilateral manual nipple massage (stimulating endogenous maternal pituitary oxytocin release) for 2 minutes or until a contraction begins, resting for 5 minutes, and repeating as needed.
- Dilute Intravenous Oxytocin Infusion: Initiated at 0.5 to 1.0 mU/min and titrated every 15 to 20 minutes until the target contraction frequency is established.
Clinical Interpretation of CST
| CST Result | Diagnostic Definition & Tracing Characteristics | Clinical Implication & Management |
|---|---|---|
| Negative (Normal) | No late decelerations or significant variable decelerations present with the required contraction pattern (≥3 contractions/10 min). | Highly reassuring. Negative predictive value for stillbirth within 1 week is >99%. Pregnancy may continue with routine surveillance. |
| Positive (Abnormal) | Repetitive late decelerations occurring with ≥50% of contractions, even if contraction frequency is <3 in 10 minutes. | Non-reassuring. Indicates significant uteroplacental insufficiency and fetal hypoxemia. Warrants inpatient admission, continuous monitoring, and consideration of delivery. |
| Equivocal - Suspicious | Intermittent late decelerations or significant variable decelerations occurring with <50% of contractions. | Inconclusive. Repeat the CST within 24 hours or perform a comprehensive Biophysical Profile (BPP). |
| Equivocal - Tachysystole | Late decelerations occurring in the presence of uterine contractions more frequent than every 2 minutes or lasting >90 seconds. | Invalid test. Hyperstimulation causes artificial fetal hypoxemia; repeat test once tachysystole resolves. |
| Unsatisfactory | Fewer than 3 contractions in 10 minutes or poor-quality tracing unable to be interpreted. | Uninterpretable. Reschedule or utilize an alternative testing modality (BPP). |
Absolute and Relative Contraindications to CST
Because a CST induces uterine contractions, it is strictly contraindicated in any clinical scenario where labor or myometrial contractions pose significant maternal or fetal hazard:
- Placenta Previa or Vasa Previa: Risk of catastrophic, life-threatening antepartum hemorrhage.
- Prior Classical (Vertical) Cesarean Incision or Prior Transmural Uterine Surgery: High risk of acute uterine rupture during contractions.
- Preterm Premature Rupture of Membranes (PPROM): Risk of accelerating preterm delivery or cord compression.
- High Risk for Preterm Labor / Incompetent Cervix / Cerclage in Place: Risk of precipitating irreversible preterm labor.
- Multiple Gestation: Elevated baseline risk of preterm labor and uterine overdistension.
The Biophysical Profile (BPP) and Modified BPP
The Biophysical Profile (BPP), pioneered by Manning and colleagues, combines real-time dynamic ultrasound assessment of five discrete biophysical variables. The central physiological principle of the BPP is that dynamic biophysical activities are controlled by distinct regulatory centers in the fetal brain that mature at different gestational ages and exhibit differential sensitivity to hypoxemia. In progressive hypoxemia, biophysical functions disappear in reverse order of their embryologic development:
- Fetal Breathing Movements: Controlled by the ventral respiratory center in the medulla; earliest to be abolished by acute hypoxemia and acidemia.
- Gross Body Movements: Controlled by the cerebral cortex and motor nuclei.
- Fetal Tone: Controlled by the subcortical centers and cerebral cortex; most resistant to hypoxia (last dynamic parameter to be lost in acute asphyxia).
- Amniotic Fluid Volume: Reflects chronic, long-term uteroplacental perfusion. Chronic hypoxemia triggers the fetal "diving reflex" (redistributing cardiac output away from the renal and splanchnic circulation toward the brain and heart), leading to decreased fetal renal filtration, reduced fetal urine output, and progressive oligohydramnios.
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| THE 5 BIOPHYSICAL PROFILE (BPP) PARAMETERS |
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Biophysical Variable Normal Criteria (Score = 2 Points) Abnormal (Score = 0 Points)
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1. Fetal Breathing ≥ 1 episode of continuous, rhythmic breathing of Absence of breathing or
Movements (FBM) ≥ 30 seconds duration in a 30-minute window. episode < 30 seconds.
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2. Gross Body ≥ 3 discrete body or limb movements in 30 minutes ≤ 2 discrete movements in
Movements (simultaneous limb/trunk movement counts as 1). 30 minutes.
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3. Fetal Tone ≥ 1 episode of active extension with return to Slow extension without return
flexion of fetal limb or spine, or opening/closing hand. to flexion, or flaccid limbs.
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4. Amniotic Fluid ≥ 1 pocket of fluid measuring ≥ 2.0 cm in vertical Single deepest vertical pocket
Volume (AFV) axis and ≥ 1.0 cm in horizontal axis (without cord). measuring < 2.0 cm.
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5. Reactive NST Meets gestational age criteria (≥2 accelerations of Nonreactive NST over a
15x15 if ≥32w, or 10x10 if <32w in 20-40 min). 40-minute window.
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Clinical Management Based on BPP Score
┌───────────────────────────────────────────────────────────────────────────────────────────────────────────────────┐
│ BPP SCORE CLINICAL MANAGEMENT ALGORITHM │
├──────────────┬─────────────────────────────────────────────────┬──────────────────────────────────────────────────┤
│ BPP Score │ Clinical Interpretation │ Recommended Inpatient / Obstetric Action │
├──────────────┼─────────────────────────────────────────────────┼──────────────────────────────────────────────────┤
│ **10 / 10** │ Normal, non-hypoxic, low risk of stillbirth │ Repeat testing per protocol (e.g., weekly) │
│ **8 / 10** │ Normal (with normal amniotic fluid volume) │ Repeat testing per protocol │
│ **8 / 10** │ Abnormal Fluid (Oligohydramnios: SDP <2 cm) │ Indicates chronic compromise; consider delivery │
│ **6 / 10** │ Equivocal; possible fetal asphyxia │ If ≥37 weeks or oligohydramnios: Deliver. │
│ │ │ If <37 weeks with normal fluid: Repeat in 24 hrs │
│ **4 / 10** │ High suspicion of chronic/acute fetal asphyxia │ Deliver if ≥32 weeks; intensive eval if <32 wks │
│ **0–2 / 10** │ Strong evidence of severe asphyxia / acidemia │ Deliver immediately regardless of gestational age│
└──────────────┴─────────────────────────────────────────────────┴──────────────────────────────────────────────────┘
The Modified BPP (mBPP) and Amniotic Fluid Assessment
- Modified BPP: Consists of a Nonstress Test (evaluating acute fetal oxygenation) combined with an ultrasound measurement of the Single Deepest Vertical Pocket (SDP) or Amniotic Fluid Index (AFI) (evaluating chronic uteroplacental function). If the NST is reactive and the fluid volume is normal (SDP ≥2 cm), the mBPP is considered normal.
- AFI vs. Single Deepest Vertical Pocket (SDP):
- Amniotic Fluid Index (AFI): Sum of the vertical depth of fluid in all four uterine quadrants (Normal: 5.0 to 24.0 cm; Oligohydramnios: <5.0 cm; Polyhydramnios: ≥24.0 cm).
- Single Deepest Vertical Pocket (SDP / Maximum Vertical Pocket): Vertical depth of the largest clear fluid pocket free of umbilical cord or fetal extremities (Normal: 2.0 to 8.0 cm; Oligohydramnios: <2.0 cm; Polyhydramnios: >8.0 cm).
- Clinical Evidence Consensus: ACOG and SMFM recommend using the Single Deepest Vertical Pocket (SDP) rather than AFI for diagnosing oligohydramnios because AFI overdiagnoses oligohydramnios, leading to higher rates of unnecessary labor inductions and cesarean deliveries without improving perinatal outcomes.
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Test Your Knowledge
A 30-week primigravida is undergoing an antepartum nonstress test (NST). Over a 20-minute monitoring window, the fetal heart rate baseline is 140 bpm with moderate variability. There are two accelerations: the first peaks at 152 bpm and lasts 12 seconds; the second peaks at 154 bpm and lasts 14 seconds. No decelerations are present. How should the nurse interpret and document this tracing?
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Test Your Knowledge
Which clinical condition represents an absolute contraindication to performing a Contraction Stress Test (CST)?
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Test Your Knowledge
A Biophysical Profile (BPP) is performed on a 38-week fetus of a patient with gestational hypertension. Ultrasound evaluation over 30 minutes demonstrates: no fetal breathing movements (0 points), 4 discrete limb/body movements (2 points), active flexion and extension of the hands and limbs (2 points), and a Single Deepest Vertical Pocket of amniotic fluid measuring 1.2 cm (0 points). The subsequent NST is reactive (2 points). The total BPP score is 6/10. What is the most appropriate next clinical step?
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Test Your Knowledge
When measuring amniotic fluid volume during antenatal surveillance, why do ACOG and SMFM recommend using the Single Deepest Vertical Pocket (SDP) technique rather than the four-quadrant Amniotic Fluid Index (AFI)?
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