The Umbilical Artery Doppler test uses ultrasound technology during pregnancy to measure blood flow in the umbilical cord. This assessment provides a dynamic view of the circulation between the fetus and the placenta. The umbilical cord contains two arteries and one vein, with the arteries carrying deoxygenated blood and waste products back to the placenta. Measuring the velocity and direction of this arterial flow helps clinicians assess the resistance blood encounters as it flows into the placental vascular bed.
The Doppler assessment is reserved for high-risk pregnancies. It serves as a placental function test, offering diagnostic and prognostic information for fetal health. Abnormal results indicate placental insufficiency, which may lead to conditions like fetal growth restriction (FGR) or pre-eclampsia. The test helps distinguish between a constitutionally small fetus and one whose growth is compromised.
The Purpose and Procedure of Umbilical Artery Doppler
The umbilical artery Doppler monitors fetal well-being in high-risk pregnancies. Indications for testing include suspected fetal growth restriction, maternal hypertensive disorders like pre-eclampsia, decreased fetal movement, multifetal pregnancies, and surveillance following a previous stillbirth. The goal is to identify fetuses experiencing suboptimal conditions so timely interventions can improve outcomes.
The procedure is performed transabdominally, similar to a standard ultrasound scan. The machine emits sound waves that bounce off moving red blood cells (the Doppler principle). The frequency shift is converted into a visual waveform showing the blood flow velocity over the fetal heart cycle.
The waveform is measured in a free-floating loop of the umbilical cord, away from the insertion points, as resistance varies by location. A clear tracing is obtained when the fetus is not actively moving, as movement can temporarily alter flow velocity. The resulting waveform is analyzed to calculate specific indices that quantify the resistance in the placental circulation.
Understanding the Key Measurements
The flow velocity waveform is used to calculate ratios that quantify the resistance, or impedance, in the placental circulation. These indices are preferred over absolute velocity measurements because they are independent of the ultrasound beam angle and less affected by fetal heart rate variations. The three common measurements derived are the Systolic/Diastolic (S/D) Ratio, the Resistive Index (RI), and the Pulsatility Index (PI).
The S/D ratio is calculated by dividing the peak systolic velocity (S) by the end-diastolic velocity (D). Systolic velocity is the maximum speed of blood flow during contraction, and end-diastolic velocity is the speed during relaxation. A higher S/D ratio indicates relatively low flow during the diastolic phase, reflecting increased resistance.
The Resistive Index (RI) is calculated using the formula: (Peak Systolic Velocity – End-Diastolic Velocity) / Peak Systolic Velocity. The Pulsatility Index (PI) uses the formula: (Peak Systolic Velocity – End-Diastolic Velocity) / Mean Velocity. Both RI and PI provide a numerical representation of the difference between the maximum and minimum flow velocities, scaled against a reference point.
Interpreting Normal and Abnormal Flow Patterns
Normal umbilical artery Doppler values change predictably throughout gestation. As a healthy pregnancy progresses, the placenta matures, leading to an increase in the number of blood vessels. This development naturally decreases vascular resistance in the umbilical arteries, reflected by an increase in end-diastolic flow.
In a normal fetus, the waveform shows continuous forward flow toward the placenta during both the systolic and diastolic phases. Normal values for the indices (S/D ratio, RI, and PI) decrease as the pregnancy advances. The expected normal range is defined by percentiles (nomograms) for a given gestational age, and a measurement is considered abnormal if it is above the 95th percentile.
Abnormal flow patterns occur in a progressive sequence, starting with increased resistance. This initial stage is characterized by a high S/D ratio, RI, and PI, where the end-diastolic flow is reduced but still present. This finding suggests early placental insufficiency, where the vascular bed of the placenta is not effectively accepting blood flow.
The next, more concerning stage is Absent End-Diastolic Flow (AEDF), where the forward flow velocity drops to zero during the diastolic phase. AEDF signifies severe placental failure. The most severe finding is Reversed End-Diastolic Flow (REDF), where the blood flow briefly reverses direction toward the fetus during diastole. REDF is associated with a high risk of poor outcomes, including perinatal mortality, and indicates severe compromise of the fetus.
Next Steps: Clinical Management Based on Results
Clinical management following an umbilical artery Doppler result depends on the abnormality’s severity and the fetus’s gestational age. When indices show only increased resistance with reduced but positive end-diastolic flow, the action is intensified fetal surveillance. This involves frequent monitoring, such as weekly biophysical profiles and non-stress tests, to track fetal health.
For severe findings, such as Absent or Reversed End-Diastolic Flow (AEDF or REDF), immediate hospital admission for continuous observation is often required. These findings necessitate a comprehensive assessment, including ductus venosus and middle cerebral artery Dopplers, to check for fetal adaptation. Management balances the risks of continuing the pregnancy against the risks of preterm delivery.
If REDF is detected, especially before term, it often leads to a decision for planned or emergent delivery, as the risk of stillbirth is significant. If AEDF is diagnosed early, intensive inpatient surveillance may be attempted. Delivery is triggered by signs of worsening fetal status, such as a drop in the biophysical profile score or the development of oligohydramnios.