20.2 Fetal & Uteroplacental Doppler

Doppler Velocimetry in Fetal Surveillance

Doppler ultrasound interrogates blood flow in the fetal and uteroplacental circulations and is central to managing growth-restricted and anemic fetuses. Waveforms are characterized by indices of downstream resistance: the systolic/diastolic (S/D) ratio, the resistive index (RI) = (peak systole minus end diastole) divided by peak systole, and the pulsatility index (PI).

Umbilical artery Doppler

The umbilical artery is sampled in a free loop of cord during fetal apnea. The placental vascular bed is normally low resistance, and as tertiary stem villi proliferate, diastolic flow rises through gestation: the S/D ratio falls from about 4 at 20 weeks to less than 3.0 after 30 weeks. An S/D ratio persistently above 3.0 (or above the 95th percentile for gestational age) after 30 weeks signals placental insufficiency - elevated resistance in the placental bed. The classic progression of worsening disease is:

1. Elevated S/D ratio, RI, and PI
2. Absent end-diastolic velocity (AEDV)
3. Reversed end-diastolic velocity (REDV)

REDV indicates that roughly three quarters or more of the placental villous vasculature is compromised; it is strongly associated with fetal growth restriction, acidemia, and perinatal mortality, and it typically triggers hospitalization, antenatal corticosteroids, intensified surveillance, and delivery planning based on gestational age. Umbilical artery Doppler is the single Doppler vessel proven to reduce perinatal death in suspected FGR, and - an exam favorite - it is not recommended as a screening tool in low-risk pregnancies.

Middle cerebral artery Doppler

The middle cerebral artery (MCA) is sampled at its proximal segment as it leaves the circle of Willis, with the angle of insonation kept near zero. The fetal brain is normally a high-resistance bed with relatively little diastolic flow. Two distinct phenomena matter for the exam:

• Brain-sparing (redistribution): fetal hypoxemia triggers cerebral vasodilation, increasing diastolic flow and decreasing the MCA RI and PI. Because umbilical artery resistance is rising at the same time, the cerebroplacental ratio (CPR) - MCA PI divided by umbilical artery PI - falls below 1 (some protocols use 1.08) when redistribution is present.
• Fetal anemia: anemic blood is less viscous and cardiac output rises, so the MCA peak systolic velocity (PSV) increases. An MCA-PSV above 1.5 multiples of the median (MoM) for gestational age predicts moderate-to-severe anemia with high sensitivity and is the standard noninvasive screen in Rh isoimmunization, parvovirus B19 infection, and suspected fetomaternal hemorrhage, largely replacing serial amniocentesis for delta OD 450 analysis.

Worked example. At 26 weeks, the median MCA-PSV is roughly 34 cm/s, so 1.5 MoM is about 51 cm/s. A measured MCA-PSV of 55 cm/s therefore flags the fetus as likely anemic and prompts referral for cordocentesis and possible intrauterine transfusion - whereas the same velocity would be unremarkable late in the third trimester, because the median PSV climbs steadily with gestational age.

Ductus venosus Doppler

The ductus venosus shunts oxygenated umbilical venous blood toward the right atrium and produces a triphasic waveform that is forward throughout the cardiac cycle: the S wave (ventricular systole), the D wave (early diastolic filling), and the a-wave (atrial contraction). An absent or reversed a-wave reflects elevated right atrial pressure from cardiac decompensation and marks severe, often pre-acidemic compromise; it is one of the parameters used to time delivery in early-onset FGR. In the first trimester, a reversed ductus venosus a-wave is also associated with trisomy 21 and congenital heart defects, particularly when the nuchal translucency is enlarged.

Uterine artery Doppler

Uterine artery waveforms are obtained transabdominally or transvaginally at 11-14 weeks and again at 20-24 weeks. Normal placentation requires trophoblastic remodeling of the spiral arteries into low-resistance vessels, so the physiologic early diastolic notch disappears by about 24 weeks. Persistence of a bilateral notch and/or an elevated RI or PI beyond 24-26 weeks identifies failed spiral artery conversion and predicts preeclampsia, fetal growth restriction, and placental abruption - especially early-onset disease. A normal second-trimester uterine artery study carries a high negative predictive value.

Doppler safety: ALARA and the output indices

Spectral, color, and power Doppler deposit substantially more acoustic energy than B-mode imaging, and the first-trimester embryo is the most heat-sensitive target in obstetric scanning. Follow ALARA - as low as reasonably achievable:

• Watch the on-screen thermal index (TI), the estimated worst-case temperature rise, and the mechanical index (MI), the likelihood of cavitation-type nonthermal effects. Keep the TI as low as possible and generally at or below 1.0; many guidelines advise a TI of 0.7 or less for first-trimester obstetric scanning.
• Minimize dwell time - do not park the Doppler beam over the embryonic heart or spine.
• In the first trimester, prefer M-mode (a low-output mode) to document cardiac activity rather than pulsed Doppler; reserve spectral Doppler for a clear clinical indication.
• Use the lowest output power and the shortest exam time that answer the question; increase receiver gain rather than output power when the signal is weak.

The displayed indices assume soft-tissue propagation at 1540 m/s; because bone heats far more readily than soft tissue, the fetal bone thermal model (TIB) governs output limits later in pregnancy.