# Obstetric Doppler Assessment

## Introduction

Doppler ultrasonography is an essential surveillance tool in high-risk obstetrics, providing non-invasive assessment of fetal, placental, and maternal hemodynamics. By evaluating blood flow velocity waveforms in key vessels, clinicians can identify fetal compromise before overt distress occurs. Doppler assessment is particularly valuable in managing fetal growth restriction, twin-to-twin transfusion syndrome, and fetal anemia, guiding delivery timing to optimize neonatal outcomes.

## Principles of Doppler Ultrasound

The Doppler effect describes the change in frequency of reflected ultrasound waves from moving red blood cells, proportional to blood flow velocity. Pulsed-wave Doppler samples flow at a specific depth and is used for most obstetric applications. Color Doppler maps flow direction and velocity in real time. The key spectral Doppler indices are the systolic/diastolic (S/D) ratio, the resistance index (RI), and the pulsatility index (PI). The PI is most widely used because it remains applicable even when diastolic flow is absent or reversed.

## Umbilical Artery Doppler

The umbilical artery (UA) reflects downstream placental vascular resistance and is the most extensively studied Doppler parameter in obstetrics. Normally, continuous forward diastolic flow is present, with S/D ratio and PI decreasing as gestational age advances and placental resistance falls. Abnormal waveforms indicate progressive placental dysfunction: elevated PI reflects increased placental vascular resistance; absent end-diastolic velocity (AEDV) indicates obliteration of approximately 60-70% of placental vasculature with significant fetal hypoxemia; and reversed end-diastolic velocity (REDV) is the most ominous finding, associated with loss of more than 70% of vasculature and perinatal mortality up to 40%.

UA Doppler is indicated for fetal growth restriction, preeclampsia, chronic hypertension, and prior stillbirth. The GRIT trial and multiple meta-analyses demonstrate that UA Doppler surveillance in high-risk pregnancies reduces perinatal mortality by approximately 29%.

<image>Spectral Doppler waveform illustration comparing normal umbilical artery flow with continuous diastolic flow, absent end-diastolic velocity, and reversed end-diastolic velocity, with labeled systolic and diastolic components</image>

## Middle Cerebral Artery Doppler

### Assessment of Fetal Anemia

MCA peak systolic velocity (MCA-PSV) is the primary non-invasive screening tool for fetal anemia, replacing cordocentesis for initial assessment. The MCA is sampled near its proximal third (closest to the internal carotid artery) with an insonation angle as close to 0 degrees as possible. MCA-PSV greater than 1.5 multiples of the median (MoM) for gestational age predicts moderate to severe anemia with approximately 100% sensitivity and 12% false-positive rate. Indications include Rh alloimmunization, Kell sensitization, and parvovirus B19 infection. Accuracy decreases after 35 weeks, and the MCA-PSV/hemoglobin relationship is altered after fetal transfusion.

### Cerebroplacental Ratio and Brain Sparing

The cerebroplacental ratio (CPR) divides MCA PI by UA PI, integrating information from both fetal and placental circulations. In response to hypoxemia, the fetus redistributes cardiac output preferentially to the brain (brain sparing), manifesting as decreased MCA PI with elevated UA PI. A CPR below the 5th percentile is associated with adverse outcomes including emergency cesarean for fetal distress, acidemia, and NICU admission. CPR is increasingly used in late-onset FGR and appropriately grown fetuses with suspected placental dysfunction.

## Ductus Venosus Doppler

The ductus venosus (DV) shunts oxygenated blood from the umbilical vein directly to the right atrium, bypassing hepatic sinusoids. Its waveform reflects fetal cardiac function and central venous pressure. The normal triphasic waveform has an S-wave (ventricular systole), D-wave (early diastole), and a-wave (atrial contraction), with forward flow throughout. Increased DV PI reflects elevated right atrial pressure, while absent or reversed a-wave indicates severe cardiac dysfunction -- a late finding in early-onset FGR that is associated with imminent fetal demise or acidemia. The DV is the key determinant of delivery timing in early-onset FGR before viability.

<image>Illustration of ductus venosus Doppler waveforms showing normal triphasic flow with forward a-wave alongside an abnormal waveform with reversed a-wave, with S-wave, D-wave, and a-wave components labeled</image>

## Uterine Artery Doppler

Uterine artery Doppler assesses maternal spiral artery remodeling and uteroplacental perfusion. Normal trophoblast invasion converts spiral arteries from high- to low-resistance vessels, reflected by decreasing PI and loss of the early diastolic notch by 24 weeks. Elevated PI (greater than 95th percentile) and persistent bilateral notching at 20-24 weeks are associated with 6-fold increased risk of preeclampsia and 4-fold increased risk of FGR. First-trimester uterine artery PI combined with maternal factors, MAP, PAPP-A, and PlGF forms the Fetal Medicine Foundation preeclampsia screening algorithm, identifying approximately 90% of early-onset preeclampsia and guiding aspirin prophylaxis.

## Sequential Doppler Deterioration in FGR

Early-onset FGR typically progresses through: elevated UA PI with normal MCA, then brain sparing (decreased MCA PI, reduced CPR), then AEDV in the UA, then REDV, then abnormal DV (elevated PI, absent/reversed a-wave), and finally abnormal biophysical profile with spontaneous decelerations. Late-onset FGR (after 32 weeks) may not follow this pattern, with CPR being the most sensitive early marker while UA Doppler remains normal. Delivery timing in early-onset FGR is guided by the TRUFFLE study: DV abnormalities prompt delivery after corticosteroids when gestational age permits.

| Doppler Parameter | What It Reflects | Abnormal Finding | Clinical Significance | Delivery Consideration |
|---|---|---|---|---|
| Umbilical artery PI | Placental vascular resistance | Elevated PI (>95th percentile) | Increased placental resistance | Intensify surveillance |
| UA - AEDV | ~60-70% placental obliteration | Absent end-diastolic flow | Significant fetal hypoxemia | Deliver ≥34 weeks; corticosteroids <34 weeks |
| UA - REDV | >70% placental obliteration | Reversed end-diastolic flow | Imminent fetal compromise; 40% perinatal mortality | Deliver ≥28-30 weeks after steroids |
| MCA-PSV | Fetal anemia | >1.5 MoM | Moderate-severe anemia (100% sensitivity) | Cordocentesis/transfusion |
| Cerebroplacental ratio | Fetal redistribution | <5th percentile | Brain sparing; adverse outcomes | Deliver ≥37 weeks; intensify surveillance earlier |
| Ductus venosus | Cardiac function/central venous pressure | Absent/reversed a-wave | Pre-terminal; severe cardiac dysfunction | Deliver after corticosteroids regardless of GA |
| Uterine artery | Uteroplacental perfusion | Elevated PI + bilateral notching | 6× risk preeclampsia; 4× risk FGR | Screening tool; initiate aspirin prophylaxis |

<image>Flowchart illustrating the sequential Doppler deterioration pattern in early-onset fetal growth restriction, from elevated umbilical artery PI through brain sparing, absent and reversed end-diastolic velocity, to ductus venosus abnormalities, with corresponding management actions at each stage</image>

## Clinical Pearls

UA Doppler is the only Doppler parameter with Level I evidence demonstrating reduction in perinatal mortality when used for surveillance of high-risk pregnancies. MCA-PSV greater than 1.5 MoM predicts moderate-severe fetal anemia with near-100% sensitivity and is the first-line non-invasive test. The CPR is more sensitive than either UA or MCA Doppler alone for detecting fetal compromise, particularly in late-onset FGR. Reversed a-wave in the ductus venosus is a pre-terminal finding that typically necessitates delivery after corticosteroid administration. UA Doppler should not be used for surveillance of normally grown fetuses -- it has no proven benefit in low-risk populations.

## References

1. Alfirevic Z, Stampalija T, Dowswell T. Fetal and umbilical Doppler ultrasound in high-risk pregnancies. *Cochrane Database Syst Rev*. 2017;6:CD007529.
2. Mari G, Deter RL, Carpenter RL, et al. Noninvasive diagnosis by Doppler ultrasonography of fetal anemia due to maternal red-cell alloimmunization. *N Engl J Med*. 2000;342(1):9-14.
3. Lees CC, Marlow N, van Wassenaer-Leemhuis A, et al. 2-year neurodevelopmental and intermediate perinatal outcomes in infants with very preterm fetal growth restriction (TRUFFLE): a randomised trial. *Lancet*. 2015;385(9983):2162-2172.
4. Defined Doppler indices for clinical use in obstetrics. Society for Maternal-Fetal Medicine (SMFM) Clinical Guideline No. 11. *Am J Obstet Gynecol*. 2012;206(4):287-296.