Residency · Residency · Obstetrics Gynecology
Routine Antepartum Surveillance and Fetal Testing
Overview
Antepartum fetal surveillance aims to identify fetuses at risk for hypoxic injury or death in time to intervene. The underlying premise is that a fetus exposed to chronic or acute hypoxemia will demonstrate detectable changes in heart rate patterns, movement, tone, and amniotic fluid volume. No single modality has been proven to reduce perinatal mortality in large randomized controlled trials, but observational evidence and physiologic rationale strongly support their use. Importantly, the false-negative rate of antepartum testing is very low -- the risk of stillbirth within one week of a reactive non-stress test is approximately 0.3%.
Non-Stress Test (NST)
Technique and Interpretation
The non-stress test uses external fetal heart rate (FHR) monitoring and a tocodynamometer, typically recorded for 20 minutes. The patient should be positioned in a semi-recumbent or lateral position to avoid aortocaval compression from the gravid uterus.
Reactive NST Criteria
A reactive NST requires two or more accelerations within 20 minutes. At 32 weeks and beyond, an acceleration is defined as an increase of at least 15 bpm lasting at least 15 seconds. Before 32 weeks, the threshold is lower: an increase of at least 10 bpm lasting at least 10 seconds. A reactive tracing should also show an absence of significant decelerations and moderate baseline variability.
Non-Reactive NST
A non-reactive NST is one that fails to meet reactive criteria within 40 minutes, even after vibroacoustic stimulation if needed. The most common cause of non-reactivity is a fetal sleep cycle, which typically lasts 20 to 40 minutes. Other causes include prematurity, maternal medications (opioids, magnesium, beta-blockers), fetal CNS anomalies, and fetal hypoxia or acidemia. A non-reactive NST warrants further evaluation, including extended monitoring, vibroacoustic stimulation, or a biophysical profile.
Vibroacoustic Stimulation (VAS)
VAS involves applying an acoustic device to the maternal abdomen over the fetal head for 1 to 3 seconds. It provokes a fetal startle response and acceleration, reducing testing time and the false non-reactive rate. VAS is safe, with no evidence of fetal hearing damage.
Biophysical Profile (BPP)
Components (2 points each, total score 0-10)
| Component | Normal (2 points) | Abnormal (0 points) |
|---|---|---|
| NST | Reactive | Non-reactive |
| Fetal breathing movements | >= 1 episode >= 30 seconds in 30 min | Absent |
| Fetal movement | >= 3 discrete body/limb movements in 30 min | < 3 movements |
| Fetal tone | >= 1 episode of flexion/extension with return | Absent or slow return |
| Amniotic fluid volume | Single deepest pocket >= 2 cm | SDP < 2 cm |
BPP Score Interpretation
A score of 8 to 10 with normal fluid is considered normal and warrants repeat testing per clinical indication. A score of 8 out of 10 with oligohydramnios should prompt delivery if the pregnancy is at 37 weeks or beyond, with further evaluation if preterm. A score of 6 out of 10 is equivocal; clinicians should consider repeating the test in 24 hours or delivering if the pregnancy is at 37 weeks or beyond, with delivery indicated if oligohydramnios is present. A score of 4 out of 10 is abnormal and should generally prompt delivery at 32 weeks or beyond, with intensive monitoring if preterm. A score of 2 or 0 out of 10 is abnormal and warrants delivery regardless of gestational age, provided the fetus is viable.
Physiologic Basis
The BPP variables are lost in the reverse order of their developmental acquisition as chronic hypoxia progresses. NST reactivity and fetal breathing movements are lost first, as they are the most sensitive to acute hypoxia. Tone and movement are lost last. Amniotic fluid volume reflects chronic uteroplacental function through its relationship to fetal renal perfusion.
Modified Biophysical Profile (mBPP)
The modified BPP combines the NST (an acute marker) with amniotic fluid assessment using either the amniotic fluid index (AFI) or single deepest pocket (SDP) as a chronic marker. It is less time-consuming than the full BPP. A normal result consists of a reactive NST with an SDP of 2 cm or more (or an AFI of 5 cm or more). An abnormal result -- a non-reactive NST or oligohydramnios -- should prompt a full BPP or further evaluation. The modified BPP is widely used as the initial screening modality in many centers.
Contraction Stress Test (CST) / Oxytocin Challenge Test (OCT)
The contraction stress test assesses the fetal response to uterine contractions, simulating labor stress. It requires three contractions in 10 minutes, achieved either spontaneously, through oxytocin administration, or via nipple stimulation. A negative result (no late decelerations) is reassuring and warrants repeat testing in one week. A positive result (late decelerations with 50% or more of contractions) is concerning for uteroplacental insufficiency. Equivocal-suspicious results show intermittent late decelerations, while equivocal-hyperstimulatory results show late decelerations only with tachysystole. An unsatisfactory result occurs when fewer than three contractions are achieved in 10 minutes. Contraindications include preterm labor risk, preterm premature rupture of membranes, placenta previa, and prior classical cesarean. The CST is rarely performed in modern practice due to time, cost, and the availability of alternatives.
Amniotic Fluid Assessment
Methods
The single deepest pocket (SDP) is preferred by many practitioners. Normal is 2 cm or more, and oligohydramnios is diagnosed when the SDP is less than 2 cm. The amniotic fluid index (AFI) sums the deepest pockets in four quadrants; normal is 5 to 24 cm, oligohydramnios is less than 5 cm, and polyhydramnios exceeds 24 cm. SDP may result in fewer interventions for oligohydramnios without worsening outcomes compared to AFI.
Clinical Significance
Oligohydramnios is associated with intrauterine growth restriction, renal anomalies, preterm premature rupture of membranes, uteroplacental insufficiency, and post-term pregnancy. Polyhydramnios is associated with gestational diabetes, fetal anomalies (particularly gastrointestinal atresias and CNS anomalies), fetal hydrops, and idiopathic causes, which account for approximately 60% of cases.
Indications and Timing of Antepartum Testing
Common Indications
Antepartum testing is indicated for hypertensive disorders (gestational hypertension, chronic hypertension, preeclampsia), pregestational and insulin-requiring gestational diabetes, fetal growth restriction, post-term pregnancy (41 weeks or beyond), decreased fetal movement, complicated multiple gestations, oligohydramnios or polyhydramnios, prior stillbirth, cholestasis of pregnancy, chronic renal disease, systemic lupus erythematosus, antiphospholipid syndrome, sickle cell disease, and poorly controlled hyperthyroidism.
General Timing Guidelines
For most conditions, testing begins at 32 weeks. Earlier initiation at 26 to 28 weeks is appropriate for severe intrauterine growth restriction, pregestational diabetes with vascular complications, and patients with multiple risk factors or a prior early stillbirth. Testing frequency is typically once or twice weekly depending on the clinical scenario: twice-weekly testing is generally used for post-term pregnancies, insulin-requiring gestational diabetes, intrauterine growth restriction, and oligohydramnios, while weekly testing suffices for most other indications.
Fetal Movement Counting (Kick Counts)
Fetal movement counting is a patient-reported monitoring tool that begins at 28 weeks in high-risk pregnancies, with some providers recommending it universally. Various protocols exist, with 10 movements in 2 hours serving as one common threshold. Decreased fetal movement warrants evaluation with a non-stress test, with or without a biophysical profile. Maternal perception of decreased movement is one of the earliest signs of fetal compromise.
<image> A medical illustration showing the setup and interpretation of the non-stress test. The upper portion shows a pregnant patient in left lateral tilt position with external monitors placed on the abdomen (Doppler transducer over fetal heart and tocodynamometer over fundus). The lower portion shows three FHR tracing strips: (1) a reactive tracing with labeled accelerations meeting criteria, (2) a non-reactive tracing with minimal variability and no accelerations, and (3) a tracing with variable decelerations. Each tracing has annotations pointing out key features. </image>
<image> A grid-style illustration of the five biophysical profile components. Each cell shows an ultrasound representation of the parameter being assessed: (1) NST tracing strip showing reactive pattern, (2) fetal chest wall movement depicting breathing movements, (3) fetal limb and trunk movements showing gross body movement, (4) fetal hand opening and closing demonstrating tone, (5) amniotic fluid pocket measurement with caliper placement. Each cell includes the scoring criteria (2 points normal, 0 points abnormal) and the physiologic basis for what it assesses. </image>
<image> A clinical algorithm flowchart for antepartum surveillance decision-making. The flowchart begins with "Identified indication for antepartum testing" and branches by gestational age and indication to determine: which test to use (NST, mBPP, full BPP, or CST), how frequently to test, and what actions to take for normal vs. abnormal results. Abnormal result pathways show escalation steps including repeat testing, further evaluation, and delivery decision-making based on gestational age. </image>
Key Clinical Pearls
A reactive NST is reassuring for approximately one week, with a false-negative rate (stillbirth within one week of a reactive NST) of approximately 1.9 per 1,000. The most common cause of a non-reactive NST is a fetal sleep cycle -- monitoring should always be extended to at least 40 minutes before declaring a tracing non-reactive. Oligohydramnios on a BPP is the most concerning single abnormal parameter because it reflects chronic placental insufficiency. Clinicians should not rely solely on the amniotic fluid index, as SDP may be more clinically appropriate and leads to fewer unnecessary interventions. A BPP score of 6 out of 10 is the most challenging to manage and demands careful consideration of clinical context and gestational age. Antepartum testing does not prevent all stillbirths, since acute events such as abruption or cord accidents may not be predicted by surveillance. Documentation should include the indication, gestational age, test result, and the plan based on the result.
References
- ACOG Practice Bulletin No. 229: Antepartum Fetal Surveillance (2021)
- Manning FA et al. Fetal Biophysical Profile Scoring: A Prospective Study in 1184 High-Risk Patients. Am J Obstet Gynecol. 1981;140:289-294
- Lalor JG et al. Biophysical Profile for Fetal Assessment in High-Risk Pregnancies. Cochrane Database Syst Rev. 2008
- Nabhan AF, Abdelmoula YA. Amniotic Fluid Index vs. Single Deepest Vertical Pocket as a Screening Test for Preventing Adverse Pregnancy Outcome. Cochrane Database Syst Rev. 2008
