The Umbilical Artery Doppler (UAD) test is a specialized, non-invasive ultrasound technique performed during pregnancy. It uses sound waves to measure blood flow within the umbilical cord, the connection between the fetus and the placenta. The test’s primary function is to assess fetal health by evaluating how well the placenta is working, specifically its ability to transfer oxygen and nutrients. Analyzing the blood flow characteristics provides insight into placental function, which acts as the fetus’s temporary lung and digestive system.
How Umbilical Artery Doppler Works
The UAD employs the Doppler effect, a principle where sound waves change frequency when they bounce off moving objects, in this case, red blood cells within the blood vessels. The ultrasound machine sends sound waves into the abdomen and then measures the returning echoes to determine the speed and direction of the blood flow through the umbilical arteries. These arteries carry deoxygenated blood and waste products away from the fetus and toward the placenta for exchange with the maternal blood supply.
The test focuses on measuring the vascular resistance, or impedance, within the placental circulation. In a healthy pregnancy, the placental blood vessels are wide and offer low resistance to blood flow, which allows for continuous, easy blood movement throughout the entire cardiac cycle. The UAD translates this flow into a waveform, where a normal pattern shows robust forward flow not just when the fetal heart beats (systole) but also when it rests (diastole). High resistance, however, is reflected as a decrease in this end-diastolic flow.
Indications for the Procedure
The UAD is not a routine screening test for every pregnancy but is reserved for situations where there is an increased risk of placental insufficiency. A common reason for ordering the procedure is suspected Intrauterine Growth Restriction (IUGR), a condition where the fetus is smaller than expected for its gestational age. This suspected growth restriction suggests the fetus may not be receiving adequate resources from the placenta.
Other indications include maternal conditions that can compromise placental function, such as preeclampsia, chronic hypertension, or pre-existing severe diabetes. A provider may also recommend the test if the mother is carrying multiples, particularly monochorionic twins who share a placenta. The test is also used if there is a history of a previous stillbirth or a fetus with poor growth.
Interpreting the Blood Flow Results
Results from the UAD are typically quantified using the Systolic/Diastolic (S/D) ratio, which compares the blood flow velocity during the peak of the fetal heart’s contraction (systole) to the velocity during its relaxation phase (diastole). A normal result shows a low S/D ratio because the placenta offers little resistance, allowing for a healthy amount of forward flow during diastole. This ratio naturally decreases as the pregnancy progresses and the placental vascular bed matures.
An elevated S/D ratio is the first sign of abnormal placental function, indicating increased resistance in the placental vessels and reduced end-diastolic flow. As placental damage progresses, the flow during the diastolic phase may completely disappear, a condition known as Absent End-Diastolic Flow (AEDF). This finding suggests a significant compromise, often with over 60% of the placental vessels being blocked.
The most concerning finding is Reversed End-Diastolic Flow (REDF), where the blood briefly flows backward from the placenta toward the fetus during diastole. REDF signifies a severe increase in placental resistance and is associated with a significantly higher risk of fetal distress and mortality.
Management Following Abnormal Findings
The clinical response to an abnormal UAD result depends entirely on the severity of the finding and the gestational age of the fetus. An elevated S/D ratio that is only mildly abnormal often leads to increased fetal surveillance, not immediate delivery. This surveillance typically includes frequent non-stress tests and biophysical profiles, which monitor the fetus’s heart rate, movement, and breathing patterns.
If the UAD shows Absent or Reversed End-Diastolic Flow, it signals that the fetus is under significant stress due to severe placental dysfunction. In these cases, the management goal shifts to balancing the risks of continued growth restriction and potential stillbirth against the risks of preterm delivery. Early delivery may be recommended if the intrauterine environment is considered more hazardous than the outside world, especially after the fetus has reached a viability threshold.
The team will closely monitor the fetus for any further deterioration in its condition or other signs of distress. Delivery is often expedited if the abnormality progresses or if other monitoring tests become non-reassuring, indicating that the fetus is no longer tolerating the compromised placental function.