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1. Umbilical Artery (UA) Doppler
Reflects placental vascular resistance. As placental obliteration progresses, end-diastolic velocity decreases:
Normal UA: High diastolic flow, low pulsatility index (PI < 95th percentile).
Elevated PI (> 95th percentile): Indicates placental insufficiency (>30% vascular bed obliteration).
Absent End-Diastolic Velocity (AEDV): Indicates critical compromise (~50% vascular bed obliterated).
Reversed End-Diastolic Velocity (REDV): Severe compromise with extreme risk of fetal acidosis and intrauterine demise.
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2. Middle Cerebral Artery (MCA) Doppler & Brain-Sparing
Assessed at the base of the skull with an insonation angle < 15°. Low MCA-PI (< 5th percentile) indicates vasodilation and brain-sparing redistribution.
Cerebroplacental Ratio (CPR = MCA-PI / UA-PI): Most sensitive parameter in late-onset FGR.
CPR < 1.0 indicates fetal redistribution even when individual UA-PI is within normal range.
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3. Ductus Venosus (DV) as the Final Arbiter
The ductus venosus reflects right atrial pressure and myocardial contractility. An absent or reversed 'a-wave' reflects metabolic acidosis and high risk of stillbirth, serving as the primary trigger for delivery in early FGR < 32 weeks.
DV a-wave reversal mandates delivery preparation with corticosteroids and magnesium sulfate.
Standardized staging: Stage 1 (mild placental), Stage 2 (severe placental/AEDV), Stage 3 (advanced/REDV or MCA vasodilation), Stage 4 (pre-terminal/DV a-wave reversed).