1.3 Fetal Assessment, Antenatal Testing & Prenatal Diagnosis
Key Takeaways
- A reactive NST at 32 weeks or later has at least two accelerations of 15 bpm for 15 seconds within the testing period; use gestational-age-appropriate criteria before 32 weeks.
- BPP and modified BPP results must be interpreted with gestational age, fluid, indication, and the overall maternal-fetal picture rather than a rigid delivery rule.
- A positive CST has late decelerations after at least half of contractions, prompting obstetric evaluation; do not provoke contractions when labor or vaginal birth is contraindicated.
- Cell-free DNA and serum analyte tests are screening tests, while CVS and amniocentesis obtain diagnostic material; a positive screen requires counseling and diagnostic options.
- Umbilical-artery and middle-cerebral-artery Dopplers support surveillance of placental resistance and fetal anemia risk, but findings require specialist interpretation.
1.3 Fetal Assessment & Antepartum Diagnostic Testing
Antepartum fetal surveillance evaluates uteroplacental function and fetal oxygenation to prevent intrauterine fetal demise and avoid unnecessary premature intervention. Inpatient obstetric nurses must be expert in executing and interpreting biophysical testing, understanding the physiologic hierarchy of fetal responses to hypoxia.
Physiological Hierarchy of Fetal Responses to Hypoxia
Fetal biophysical activities develop at specific gestational ages as the central nervous system matures. When hypoxia occurs, biophysical activities disappear in the reverse order of their embryological development:
- Fetal Heart Rate Reactivity (NST) & Fetal Breathing: Controlled by the hypothalamus and medulla; sensitive to early, mild acidemia (disappear first).
- Gross Body Movements & Fetal Tone: Controlled by cortex and subcortical nuclei; disappear with progressive moderate acidemia.
- Amniotic Fluid Volume: Reflected by fetal renal perfusion; decreases with chronic hypoxia due to compensatory redistribution of cardiac output to the fetal brain, heart, and adrenal glands (oligohydramnios occurs over days/weeks).
Nonstress Test (NST) & Vibroacoustic Stimulation (VAS)
The NST assesses fetal heart rate acceleration in response to fetal movement or autonomic tone.
Reactive vs. Nonreactive Criteria
- >= 32 Weeks Gestation: A Reactive NST requires >= 2 accelerations of the fetal heart rate reaching a peak of at least 15 bpm above baseline and lasting for at least 15 seconds (15 x 15 rule) within a 20-minute window.
- < 32 Weeks Gestation: A Reactive NST requires >= 2 accelerations reaching at least 10 bpm above baseline and lasting for at least 10 seconds (10 x 10 rule) within a 20-minute window.
- Nonreactive NST: Lacks adequate accelerations over a 40-minute testing period (accounting for the normal 20- to 40-minute fetal sleep cycle).
Vibroacoustic Stimulation (VAS)
If the initial 20-minute NST window lacks accelerations, VAS may be applied to the maternal abdomen over the fetal head:
- Device Application: Apply artificial larynx / acoustic stimulator for 1 to 3 seconds.
- Repetition: May repeat at 1-minute intervals up to 3 times.
- Reassuring Response: An acceleration after stimulation supports fetal responsiveness and can shorten testing, but it does not independently prove the absence of metabolic acidemia.
Biophysical Profile (BPP) Scoring System
The Manning Biophysical Profile combines real-time ultrasound evaluation of four fetal biophysical variables with the Nonstress Test over a maximum observation period of 30 minutes.
BPP Parameter Scoring (0 or 2 Points Each)
| BPP Variable | Normal Criteria (Score = 2) | Abnormal Criteria (Score = 0) |
|---|---|---|
| Nonstress Test (NST) | Reactive NST (>= 2 accelerations in 20–40 min) | Nonreactive NST (< 2 accelerations) |
| Fetal Breathing Movements (FBM) | >= 1 episode of sustained rhythmic breathing lasting >= 30 seconds within 30 minutes | Absence of breathing or < 30 seconds of continuous breathing |
| Fetal Gross Body Movement | >= 3 discrete body or limb movements within 30 minutes (simultaneous roll/limb movement counts as one) | < 3 discrete body/limb movements |
| Fetal Muscle Tone | >= 1 episode of active extension of a fetal limb or spine with return to flexion (opening and closing of hand) | Slow extension with partial return to flexion, or flaccid posture |
| Amniotic Fluid Volume | >= 1 vertical pocket of fluid measuring >= 2.0 cm in two perpendicular planes (Single Deepest Pocket) | No vertical pocket measuring >= 2.0 cm (SDP < 2.0 cm) |
Interpretation & Clinical Management Principles
- 8/10 or 10/10 with normal fluid: Generally reassuring; repeat surveillance at the interval set by the indication and clinical condition.
- 6/10: Equivocal. Promptly review gestational age, fluid, testing indication, medications, maternal status, and the entire fetal assessment; repeat testing or birth may be appropriate.
- 4/10 or lower: Abnormal and requires prompt obstetric or maternal-fetal-medicine evaluation. Very low scores can support urgent birth, but prematurity risk and the complete maternal-fetal picture still matter.
- Abnormal fluid: Oligohydramnios changes management even when the other biophysical variables are reassuring. Do not use a score-only delivery rule without gestational age, membrane status, Dopplers when indicated, and maternal condition.
Modified Biophysical Profile (mBPP)
The modified BPP is a rapid, primary antepartum surveillance tool combining:
- Nonstress Test (NST): Short-term marker of fetal acid-base balance.
- Amniotic Fluid Index (AFI): Long-term marker of placental function.
- Normal mBPP: Reactive NST AND AFI > 5.0 cm (or SDP > 2.0 cm).
- Abnormal mBPP: Nonreactive NST OR low fluid prompts timely further obstetric evaluation; a full BPP, repeat testing, Dopplers, or delivery may be selected from the clinical context.
Contraction Stress Test (CST)
The CST evaluates fetal heart rate responses to induced uterine contractions, assessing uteroplacental reserve under transient hypoxia.
Testing Methodology
- Target Contraction Pattern: Requires 3 contractions within a 10-minute window, each lasting >= 40 seconds.
- Induction Methods: Spontaneous, Nipple Stimulation (roll nipple for 2 min or until contraction starts), or IV Oxytocin Infusion (Pitocin starting at 0.5 mU/min and titrating q15m).
When CST Should Not Be Performed
Do not provoke contractions when labor or vaginal birth is contraindicated, including placenta or vasa previa and a prior classical uterine incision. Preterm gestation, ruptured membranes, cervical insufficiency or cerclage, multifetal gestation, and other high-risk conditions require individualized specialist assessment rather than a blanket bedside rule.
CST Result Categories
| CST Classification | Diagnostic Criteria | Clinical Significance |
|---|---|---|
| Negative | No late or significant variable decelerations | Reassuring. Uteroplacental reserve adequate. Low mortality risk for 1 week. |
| Positive | Late decelerations following >= 50% of contractions | Nonreassuring; prompts obstetric evaluation using gestational age and the full maternal-fetal picture. |
| Equivocal-Suspicious | Intermittent late decelerations or variable decelerations | Non-diagnostic. Repeat test within 24 hours. |
| Equivocal-Tachysystole | Decelerations occurring with contractions more frequent than q2m or lasting > 90 sec | Invalid. Decelerations caused by hyperstimulation. Relax uterus; repeat test. |
| Unsatisfactory | < 3 contractions in 10 minutes, or uninterpretable FHR tracing | Technical failure. Repeat test using nipple stimulation or oxytocin. |
Amniotic Fluid Assessment: AFI vs. SDP
Amniotic fluid volume reflects fetal urine production and renal perfusion.
- Amniotic Fluid Index (AFI): Sum of the single deepest vertical pockets in each of four maternal abdominal quadrants. Normal: 5.1 to 23.9 cm.
- Single Deepest Pocket (SDP): Measurement of the deepest clear vertical pocket of fluid free of cord/limbs. Normal: 2.0 to 7.9 cm.
- Oligohydramnios: Defined as AFI <= 5.0 cm or SDP < 2.0 cm. Etiologies: Uteroplacental insufficiency, ACE inhibitor exposure, renal agenesis (Potter sequence), PPROM. Risks: Cord compression (variable decelerations), pulmonary hypoplasia.
- Polyhydramnios (Hydramnios): Defined as AFI >= 24.0 cm or SDP >= 8.0 cm. Etiologies: Maternal GDM, fetal duodenal atresia, anencephaly, hydrops fetalis. Risks: Cord prolapse upon ROM, placental abruption, uterine atony postpartum.
Ultrasound, Doppler & Prenatal Diagnosis
Ultrasound assesses anatomy, placental location, fetal number and chorionicity, presentation, fluid, growth, and selected markers. Interpret estimated fetal weight using percentile and trend with Dopplers, fluid, and the clinical indication.
Umbilical-artery Doppler evaluates placental resistance, especially in fetal growth restriction. Absent or reversed end-diastolic flow is high risk and requires maternal-fetal medicine management. Middle-cerebral-artery peak systolic velocity is a noninvasive screening and surveillance measure for fetal anemia; an elevated multiple of the median raises concern but is not a definitive diagnosis. Ductus venosus Doppler may be added in selected severe cases.
Prenatal screening estimates probability. Cell-free DNA can screen for common aneuploidies from early pregnancy and is highly sensitive for trisomy 21, but a positive result is not diagnostic. Serum screening and ultrasound remain options based on timing and preference. Maternal serum AFP contributes to open neural-tube-defect screening.
Chorionic villus sampling obtains placental tissue for diagnostic genetic testing in the first trimester but does not measure amniotic-fluid AFP. Amniocentesis obtains fluid for genetic or selected infectious testing and can assess AFP and acetylcholinesterase. Percutaneous umbilical blood sampling provides direct fetal blood for selected diagnosis or treatment at a specialty center. Before an invasive procedure, verify consent, Rh status, baseline fetal status as indicated, and postprocedure warning signs; give Rh immune globulin when indicated.
Fetal-lung-maturity amniocentesis is not routine. A “mature” result does not justify nonmedically indicated early delivery, and medically indicated delivery should not be delayed solely to obtain maturity testing.
A patient at 38 weeks gestation undergoes a Manning Biophysical Profile (BPP). Ultrasound findings demonstrate: Nonreactive NST (0 points), 2 episodes of sustained fetal breathing lasting 45 seconds (2 points), 4 discrete body movements (2 points), active arm extension with return to flexion (2 points), and a single deepest vertical fluid pocket of 1.2 cm (0 points). Total score is 6/10. What is the most appropriate management plan?
A patient at 35 weeks gestation undergoes a Contraction Stress Test (CST) using IV oxytocin. The tracing demonstrates 4 uterine contractions in a 10-minute period. Late decelerations are observed following 3 out of the 4 contractions. How should the nurse classify and report this CST result?
An ultrasound report for a patient at 34 weeks gestation indicates an Amniotic Fluid Index (AFI) of 3.8 cm. Which clinical concern should the nurse prioritize during fetal surveillance?