An umbilical artery Doppler test is a common prenatal assessment that uses sound waves to evaluate blood flow within the umbilical cord. This non-invasive ultrasound technique helps healthcare providers assess placental function and determine if the fetus is receiving sufficient oxygen and nutrients.
What is an Umbilical Artery Doppler
An umbilical artery Doppler uses specialized ultrasound technology to visualize and measure blood flow in the umbilical arteries. The umbilical cord contains two arteries that carry deoxygenated blood and waste products away from the fetus to the placenta, and one vein that delivers oxygenated, nutrient-rich blood from the placenta to the fetus. By assessing these flow patterns, healthcare providers gain information about placental function and fetal health.
Common reasons for conducting this test include concerns about fetal growth restriction, where a fetus does not grow as expected. It is also used in high-risk pregnancies, such as those complicated by pre-eclampsia, gestational hypertension, or multiple pregnancies like monochorionic twins who share a single placenta.
Understanding Normal Umbilical Artery Values
Normal umbilical artery Doppler values indicate adequate blood flow from the fetus to the placenta, suggesting proper placental function. Healthcare providers assess three key measurements: the Systolic/Diastolic (S/D) ratio, the Pulsatility Index (PI), and the Resistive Index (RI). These indices reflect the resistance to blood flow within the umbilical arteries, which changes as pregnancy progresses.
The S/D ratio is calculated by dividing the peak systolic velocity (maximum blood flow during heart contraction) by the end-diastolic velocity (blood flow during heart relaxation). A normal S/D ratio decreases with advancing gestational age, reflecting placental maturation and reduced resistance. For instance, around 20 weeks of gestation, the 50th percentile for the S/D ratio is approximately 4.0, decreasing to about 2.83 at 30 weeks, and further to around 2.18 at 40 weeks.
The Pulsatility Index (PI) and Resistive Index (RI) are other measures of vascular resistance within the umbilical arteries. Like the S/D ratio, normal PI and RI values decrease as pregnancy advances due to the development of more tertiary stem villi in the placenta, which helps lower resistance to blood flow. For example, the mean PI decreases from around 1.270 to 0.967, and the mean RI decreases from approximately 0.756 to 0.609 as pregnancy progresses. These declining values signify a healthy, low-resistance blood flow pattern that supports fetal growth and well-being.
When Readings Are Not Normal
When umbilical artery Doppler readings fall outside the expected normal ranges, it indicates increased resistance to blood flow within the umbilical arteries. This can manifest as a high S/D ratio, PI, or RI, meaning there is less blood flow during the diastolic phase of the fetal heartbeat. Such findings suggest the placenta may not be functioning optimally, a condition known as placental insufficiency.
More concerning abnormal findings include absent end-diastolic flow (AEDF), where there is no forward blood flow in the umbilical artery during the diastolic phase. Even more severe is reversed end-diastolic flow (REDF), where blood flows backward, away from the placenta, during diastole. Both AEDF and REDF are strong indicators of significant placental insufficiency and are associated with increased risks, including fetal growth restriction, fetal hypoxia, and even fetal demise.
The severity of these abnormalities correlates with the potential risk to the fetus; for example, reversal of end-diastolic flow is particularly worrisome for fetal mortality. These abnormal patterns suggest the placental vasculature may be underdeveloped or have undergone changes that increase resistance to blood flow, thereby compromising the delivery of oxygen and nutrients to the fetus.
What Influences Readings and Next Steps
Several factors can influence umbilical artery Doppler readings beyond placental health. Fetal breathing movements and fetal heart rate affect the blood flow patterns observed. Additionally, the location where the Doppler measurement is taken along the umbilical cord can impact the readings, with measurements closer to the fetal insertion showing higher resistance. Maternal conditions such as pre-eclampsia can also exacerbate abnormalities in umbilical artery Doppler readings.
Upon obtaining Doppler results, medical professionals interpret these values in conjunction with other clinical information about the pregnancy. If readings are consistently normal, routine prenatal care continues. However, if abnormal readings are detected, especially increased resistance, absent, or reversed diastolic flow, further actions are taken. These steps may include increased fetal monitoring, such as more frequent Doppler scans, non-stress tests, or biophysical profiles, to closely track fetal well-being. In cases of severe or worsening abnormalities, healthcare providers may consider further investigations or plan for early delivery to mitigate risks to the fetus, always weighing the gestational age against the severity of the findings.