Fetal Well-Being Assessment: BPP, AFI, and Doppler for Sonographers

Fetal well-being assessment is a core part of third-trimester obstetric ultrasound. New OB sonographers often feel uncertain about performing BPP correctly and interpreting Doppler waveforms. This is your practical guide to the most common fetal surveillance studies.

Why Fetal Surveillance Matters

Fetal compromise occurs when placental function declines and the fetus can no longer maintain normal oxygenation. The earliest sign is redistribution of blood flow (the "brain-sparing" response), followed by changes in fetal behavior, and ultimately fetal demise if untreated.

The tools we use — BPP, AFI, and Doppler — detect different stages of this cascade. A negative result is reassuring; an abnormal result prompts closer monitoring or delivery planning.

Biophysical Profile (BPP)

The BPP scores 5 parameters, each worth 2 points (maximum 10):

Parameter	Criteria for 2 points
Fetal breathing movements	≥ 1 episode lasting ≥ 30 seconds in 30 minutes
Gross body movements	≥ 3 discrete body/limb movements in 30 minutes
Fetal tone	≥ 1 episode of extension with return to flexion in 30 minutes
Amniotic fluid volume	≥ 1 pocket measuring ≥ 2 cm in two perpendicular planes
Non-stress test (NST)	≥ 2 accelerations of ≥ 15 bpm lasting ≥ 15 seconds in 20 minutes

Each parameter gets 2 or 0 — no partial scores.

Interpretation

Score	Interpretation	Action
8–10	Normal — low risk of asphyxia	Routine surveillance per gestational age
6	Equivocal	Repeat in 24 hours; consider delivery if ≥ 36 weeks
4	Abnormal	Delivery often recommended
0–2	Strongly abnormal	Delivery indicated

If amniotic fluid is the sole abnormality: Score 8/10 with oligohydramnios may still prompt further evaluation.

Modified BPP

The NST + AFI (without the full 4-parameter ultrasound portion) is used as a quick screening tool. It's faster and adequate for most routine surveillance situations.

Scanning the BPP

Observe the fetus for up to 30 minutes. Fetal behavioral states change in cycles — you may need to wait for the fetus to come out of a sleep cycle (normal fetal sleep cycles last 20–40 minutes).

Tips:

• Acoustic stimulation (vibroacoustic stimulator or tapping the maternal abdomen) can arouse a sleeping fetus — check if your department uses this
• Cold water or a snack for mom sometimes prompts fetal movement
• Don't count the same movement twice for multiple parameters — a single movement can count for movement AND tone simultaneously

Amniotic Fluid Index (AFI)

AFI is measured by dividing the uterus into four quadrants using the umbilicus and linea nigra.

Technique:

1. Transducer perpendicular to the floor (not to the maternal abdomen)
2. Largest vertical pocket in each quadrant, free of fetal parts and umbilical cord
3. Measure the deepest unobstructed column of fluid
4. Sum all four measurements

Normal AFI: 5.0–25.0 cm

AFI	Classification
> 25 cm	Polyhydramnios
8–25 cm	Normal
5–8 cm	Low normal / borderline
< 5 cm	Oligohydramnios

Maximum Vertical Pocket (MVP): An alternative single-pocket measurement. Normal = 2–8 cm. < 2 cm = oligohydramnios. Some institutions and protocols use MVP rather than AFI — know your department's preference.

Oligohydramnios causes: IUGR (placental insufficiency), post-dates, renal agenesis/urinary anomalies, ruptured membranes

Polyhydramnios causes: Fetal swallowing abnormalities (esophageal atresia, neurologic problems), maternal diabetes, anemia, idiopathic

Umbilical Artery Doppler

The umbilical artery (UA) Doppler is the primary Doppler tool for monitoring IUGR.

What it measures: Resistance in the feto-placental circulation. As placental function deteriorates, peripheral resistance in the placenta increases, reducing diastolic flow.

Normal waveform: Continuous forward flow throughout the cardiac cycle, with significant diastolic flow (low-resistance pattern). S/D ratio decreases normally as gestation advances.

Abnormal findings (in order of severity):

1. Elevated S/D ratio for gestational age: (> 3.0 after 30 weeks is concerning) — mildly elevated resistance
2. Absent end-diastolic flow (AEDF): Diastolic component reaches zero — significant placental dysfunction
3. Reversed end-diastolic flow (REDF): Diastolic flow reverses direction — severe, often associated with fetal acidemia

Technique:

• Free loop of cord in amniotic fluid (avoid areas near placenta or fetal abdomen for UA)
• Insonation angle ≤ 60°; many prefer 0° if achievable in free cord
• Average 3–5 consecutive consistent waveforms

Middle Cerebral Artery (MCA) Doppler

The MCA Doppler measures two things:

1. Brain-sparing response: Vasodilation of cerebral vessels to maintain brain perfusion when placental function declines
2. Fetal anemia: Elevated peak systolic velocity (PSV) due to increased cardiac output and reduced blood viscosity

MCA PSV for Anemia Screening

Used in alloimmunization (Rh disease), twin-twin transfusion syndrome, and suspected fetal anemia.

Normal: MCA PSV increases with gestational age — you must compare to the 1.5 MoM (multiples of median) curve

Threshold: MCA PSV > 1.5 MoM for gestational age = moderate to severe fetal anemia (sensitivity ~100% for severe anemia)

Technique:

• Axial view of fetal brain at the level of the thalami and cerebral peduncles
• Color Doppler: MCA is the vessel running lateral from the circle of Willis toward the temporal lobe
• Place sample gate at the proximal third of the MCA (near the circle of Willis)
• Angle correction: 0° is ideal — the MCA runs toward or away from the probe in the standard axial view
• Measure PSV (peak of systole) — do NOT angle correct or the value is invalid if near 0°

MCA Pulsatility Index for IUGR Brain-Sparing

PI (Pulsatility Index): (PSV − EDV) / mean velocity

Low MCA PI (vasodilation, increased diastolic flow) in setting of high UA PI = cerebrovascular redistribution = brain-sparing = significant fetal compromise.

Cerebroplacental ratio (CPR): MCA PI / UA PI. < 1.0 is abnormal and suggests brain-sparing.

Ductus Venosus (DV) Doppler

The DV is a small vessel connecting the umbilical vein to the IVC, bypassing the liver. It reflects cardiac preload.

Normal waveform: All-forward flow (positive during atrial systole)

Abnormal: Absent or reversed "a-wave" (during atrial contraction) = elevated venous pressure, fetal cardiac decompensation. This is a late, severe sign.

Putting It Together

The deterioration sequence in severe IUGR:

1. UA diastolic flow decreases (elevated S/D ratio)
2. MCA vasodilates (brain-sparing, low MCA PI)
3. UA diastolic flow becomes absent (AEDF)
4. UA flow reverses (REDF)
5. DV a-wave becomes absent or reversed
6. Fetal demise without intervention

Surveillance frequency and delivery timing decisions belong to MFM and OB — your role is accurate measurement and clear documentation.

Documentation Tips

For every fetal well-being study, document:

• Gestational age and estimated fetal weight
• BPP parameters (each individually with score)
• AFI (all four quadrant measurements)
• UA waveform description (S/D ratio, RI, note if AEDF or REDF)
• MCA PSV and MoM if indicated for anemia
• Placental location and appearance
• Fetal presentation

---

SonoBuddy's Calculators include AFI and gestational age calculations — use them at the bedside for quick reference.