A Middle Cerebral Artery (MCA) Doppler scan is a specialized, non-invasive ultrasound performed during high-risk pregnancies. This test measures the speed of blood flowing through the middle cerebral artery, a major blood vessel in the brain of the developing fetus. By assessing this blood flow, perinatology specialists can gather information about a fetus’s health, particularly when there is a concern for specific medical conditions.
Purpose of an MCA Doppler Scan
The primary application for an MCA Doppler scan is the detection of fetal anemia. Fetal anemia occurs when the fetus lacks enough healthy red blood cells to carry oxygen. This forces the fetal heart to pump harder and faster to compensate, which can lead to complications such as heart failure or a condition called hydrops fetalis, where excessive fluid builds up in the fetal body. The underlying principle is that anemic blood is thinner, or less viscous, and therefore flows more rapidly through blood vessels.
A physician may order this scan when certain risk factors are present. One of the most common reasons is Rh alloimmunization, a condition where a mother’s immune system attacks the fetus’s red blood cells. This incompatibility can lead to significant anemia. Another frequent indication is an infection with Parvovirus B19 during pregnancy, which can temporarily halt the fetus’s production of red blood cells, causing severe, though often transient, anemia.
The scan is also used in the management of complex twin pregnancies. In cases of Twin-to-Twin Transfusion Syndrome (TTTS), where there is an imbalanced blood flow between twins, the MCA Doppler can help monitor the fetuses for anemia or other circulatory issues. Additionally, it is used to assess fetuses with Intrauterine Growth Restriction (IUGR). In these cases, the scan can help determine if the fetus is redirecting blood flow to its brain to protect it from oxygen deprivation, a mechanism known as “brain sparing.”
The MCA Doppler Procedure
The experience of undergoing an MCA Doppler scan is very similar to a standard obstetrical ultrasound. The expectant mother lies on an examination table, and a clear, water-based gel is applied to her abdomen. This gel helps to ensure a solid connection between the skin and the ultrasound transducer for the sound waves to travel effectively.
Once the fetus is in a suitable position, the sonographer focuses on obtaining a cross-section of the fetal head to visualize the major blood vessels. Using the color Doppler function, the middle cerebral artery is identified as it branches off from the internal carotid artery. The machine then measures the velocity of the blood moving through this specific artery. The fetus must be still during the measurement to ensure an accurate reading.
Understanding the Results
The primary measurement obtained from an MCA Doppler is the Peak Systolic Velocity (PSV), which represents the fastest speed blood travels through the artery during a heartbeat. A high PSV reading can therefore be an indirect sign of fetal anemia. This finding has been shown to be a sensitive predictor of moderate to severe anemia in at-risk fetuses.
To determine if the PSV is abnormally high, the value is compared against established reference ranges for the fetus’s specific gestational age. Results are reported as a Multiple of the Median (MoM). The median is the “middle” or typical PSV value for a healthy fetus at that stage of pregnancy. A PSV value that is more than 1.5 times the median ( >1.5 MoM) is considered the threshold for concern and suggests a high probability of moderate or severe anemia.
This MoM value allows for a standardized interpretation of results across different stages of pregnancy, as the normal PSV naturally increases as the fetus grows. It is important to note that the false-positive rate can increase after 35 weeks of gestation, meaning other factors might cause a high reading.
Next Steps After an Abnormal Result
An abnormal MCA Doppler PSV result is a screening indicator, not a final diagnosis. If a reading comes back high (above 1.5 MoM), the first step is often to repeat the scan within a few days to confirm the finding and observe any trend. Consistent, significantly elevated readings will prompt a discussion about more definitive diagnostic testing to measure the fetus’s blood count.
The most accurate method for diagnosing fetal anemia is an invasive procedure called cordocentesis, also known as Percutaneous Umbilical Cord Blood Sampling (PUBS). During cordocentesis, a specialist uses ultrasound guidance to insert a thin needle through the mother’s abdomen and uterus into the umbilical cord to draw a small sample of fetal blood. This sample is analyzed to determine the hemoglobin level and confirm anemia.
If severe anemia is confirmed, the primary treatment is an intrauterine blood transfusion (IUT). In this procedure, red blood cells that are compatible with the fetus’s blood type are transfused directly into the umbilical cord vein. This intervention can correct the anemia, relieve strain on the fetal heart, and reverse conditions like hydrops fetalis, often allowing the pregnancy to continue safely. In cases involving parvovirus, a single transfusion is frequently sufficient to resolve the issue.