Fetal Doppler ultrasound is a cornerstone of high-risk obstetric surveillance. It provides physiologic information about placental resistance and fetal circulatory adaptation that biometry alone cannot give. Understanding how to perform these studies and interpret the waveforms is essential for any sonographer working in OB.

Why Fetal Doppler?

Doppler studies are ordered when:

Fetal growth restriction (FGR) is suspected or confirmed
Fetal anemia risk (isoimmunization, infection, hydrops)
Hypertensive disorders of pregnancy (preeclampsia)
Prior FGR or adverse perinatal outcome
Discordant twins (MCDA: TTTS, sFGR)
Post-dates pregnancy (placental insufficiency)

The goal: identify the compromised fetus before clinical deterioration, so delivery can be timed safely.

Umbilical Artery Doppler

The umbilical artery (UA) reflects downstream placental resistance. As placental dysfunction progresses, resistance rises — the waveform changes in a predictable sequence.

Technique

Probe: Curved low-frequency (3–5 MHz) or appropriate for fetal depth.

Gate placement: Free loop of umbilical cord, away from the fetal or placental insertion (fetal insertion is preferred in some guidelines). The waveform differs slightly at placental vs fetal end.

Angle correction: Not used for UA Doppler — S/D ratio is angle-independent (ratio of peak systole to end-diastole).

Optimal technique:

Fetus at rest (movement and breathing affect waveform)
Measure over 3–5 consecutive waveforms
Obtain the angle of insonation ≤ 60°

Normal UA Waveform

High-resistance in early pregnancy (very little or no diastolic flow), progressively lower resistance with gestation (diastolic flow increases as the placental vascular tree develops).

By 24 weeks: Clear forward diastolic flow should be present.

Normal S/D ratio: Decreases with gestation (typically < 3.0 after 30 weeks).

Abnormal UA Patterns (in order of severity)

Pattern	Significance
Elevated S/D ratio	Increased placental resistance — early FGR sign
Absent end-diastolic flow (AEDF)	Severe — 50–70% of placental vascular bed obliterated
Reversed end-diastolic flow (REDF)	Critical — imminent fetal compromise; delivery planning urgent

AEDF or REDF: Report immediately. These findings are associated with perinatal morbidity and mortality and directly trigger obstetric decision-making (steroid administration, NICU preparation, delivery planning).

Reporting note: Specify pattern, not just "abnormal." "Absent end-diastolic flow in the umbilical artery" is actionable. "Abnormal UA Doppler" is not.

Middle Cerebral Artery (MCA) Doppler — PSV

The MCA is used primarily to detect fetal anemia. Not the same indication as UA Doppler.

Why MCA PSV for Anemia?

When a fetus becomes anemic, the blood thins (decreased viscosity) and cardiac output increases — blood moves faster through all vessels. The MCA PSV (peak systolic velocity) rises as a measurable physiologic response to anemia.

MCA PSV > 1.5 MoM (multiples of the median) for gestational age = fetal anemia (moderate to severe). Sensitivity ~88%, specificity ~82% for Hgb < 0.84 MoM.

Technique — This Is Technically Demanding

Probe: Curved or sector probe.

Steps:

Obtain an axial cross-section of the fetal head at the level of the circle of Willis (same plane as BPD — thalami visible)
Locate the middle cerebral artery using color Doppler — it runs from the circle of Willis laterally toward the temporal lobe
Place the Doppler gate at the proximal third of the MCA — near the origin from the internal carotid (most reproducible)
Critical: zero angle of insonation (0°) — the MCA must run directly toward or away from the probe. Even a 10–15° angle significantly underestimates PSV. Tilt the probe to make the MCA run horizontal.
Do NOT compress the fetal head (head compression falsely elevates PSV)
Measure the PSV (peak systolic velocity) — the highest point on the waveform
Take multiple measurements and use the highest reproducible PSV (not the mean)

Normal MCA PSV Values

Values increase with gestational age. The clinical threshold is 1.5 MoM (multiples of the median) for that GA.

Reference: Mari et al., 2000 (NEJM) — the foundational reference.

Approximate normal MCA PSV medians:

Gestational Age	Median MCA PSV
20 weeks	~40 cm/s
24 weeks	~47 cm/s
28 weeks	~55 cm/s
32 weeks	~64 cm/s
36 weeks	~74 cm/s
40 weeks	~87 cm/s

1.5 MoM = 1.5 × median for GA. For 28 weeks: 1.5 × 55 = 82.5 cm/s — any PSV above this suggests anemia.

When Is MCA PSV Ordered?

Maternal red cell alloimmunization (anti-D, anti-Kell, etc.)
Prior history of severe fetal anemia or hydrops
Parvovirus B19 exposure (slapped cheek disease)
Hydrops of unknown cause
Following intrauterine transfusion (surveillance for recurrence)

MCA Pulsatility Index (MCA PI) — Cerebral Redistribution

The MCA PI is used in a different context — cerebral redistribution (brain sparing) in growth-restricted fetuses.

When a fetus is hypoxic from placental insufficiency, it prioritizes blood flow to the brain by vasodilating cerebral vessels. The MCA becomes a low-resistance vessel (PI drops).

Normal: MCA PI > UA PI (brain resistance is higher than placental resistance).

Brain sparing: MCA PI falls below the UA PI → the fetus is shunting blood to the brain at the expense of other organs. This is a sign of significant fetal compromise.

Cerebroplacental ratio (CPR): MCA PI / UA PI. CPR < 1.0 = brain sparing = concerning.

Ductus Venosus (DV) Doppler

The ductus venosus carries oxygenated blood from the umbilical vein directly to the right atrium, bypassing the liver. DV Doppler reflects right heart function and central venous pressure.

When Is It Used?

DV Doppler is a late-stage surveillance tool — used when UA AEDF/REDF is present, or in the setting of severe FGR, to assess cardiac decompensation.

Normal DV Waveform

Triphasic waveform with three components:

S wave: Ventricular systole (forward flow)
D wave: Ventricular diastole (passive forward flow)
a wave: Atrial contraction — normally forward, above the baseline

Key: Forward a-wave = normal cardiac function.

Abnormal DV Patterns

Pattern	Significance
Reduced a-wave	Elevated venous pressure — early cardiac compromise
Absent a-wave	Significant compromise — elevated right atrial pressure
Reversed a-wave	Critical — severe cardiac compromise; imminent fetal decompensation; usually indication for delivery

DV reversed a-wave in a premature fetus is a grave sign. Correlate with clinical context and biophysical profile.

Systematic Approach: The Modified Biophysical Profile and Doppler

In surveillance of the compromised fetus:

UA Doppler first — placental resistance
If UA abnormal → MCA PI — cerebral redistribution assessment
If UA AEDF/REDF → DV Doppler — cardiac venous function
Biophysical profile — amniotic fluid, movements, breathing, tone, NST

This hierarchy reflects the progression of fetal deterioration: placental → brain → heart.

Reporting Language

Normal: "Umbilical artery: normal forward end-diastolic flow. S/D ratio 2.6 (normal for 34 weeks). MCA PSV 58.2 cm/s (0.92 MoM — normal). Ductus venosus: normal triphasic waveform with forward a-wave."

Absent EDV: "Umbilical artery: absent end-diastolic flow identified in a free loop of cord at 30 weeks gestation. MCA PI 1.34 (low — brain sparing pattern). Ductus venosus: forward a-wave, mildly reduced. Umbilical artery AEDF at this gestational age represents significant placental compromise. Urgent obstetric consultation is recommended."

MCA PSV for anemia: "MCA PSV: 92 cm/s at 30 weeks gestation (1.67 MoM). Values above 1.5 MoM indicate risk for moderate to severe fetal anemia. Clinical correlation with maternal antibody titers and consideration of cordocentesis is recommended."

SonoBuddy's OB Doppler protocols include umbilical artery, MCA PSV, and ductus venosus references with gestational-age-specific MoM tables.

Fetal Doppler: MCA PSV, Umbilical Artery, and Ductus Venosus