A fetal middle cerebral arterial (MCA) Doppler is used to predict, diagnose and monitor the progression of fetal anemia. It is also useful in managing the optimal interval between intrauterine blood transfusions.
An MCA Doppler assessment can be very useful in pregnancies affected by hemolytic disease of the fetus and newborn (HDFN). It is an accurate and non-invasive procedure using ultrasound technology that assesses fetal blood flow and hypoxia.
What is HDFN?
Hemolytic disease of the fetus and newborn (HDFN) is an immune-mediated red blood cell disorder that occurs when a baby’s RBCs break down quickly, which is called hemolysis. HDFN is caused by a mismatch between a mother’s and her baby’s blood type (A, B, AB, or O) or Rhesus (Rh) factor (Rh-positive or Rh-negative) during pregnancy.
What is an MCA Doppler assessment?
An MCA Doppler assessment monitors the peak velocity of systolic blood flow in the middle cerebral artery.
It is performed like a regular ultrasound, with the help of gel and a handheld device used on the pregnant mother’s abdomen. The ultrasound technologist locates the fetus and the middle cerebral artery, which is a major vessel in the brain and measures the speed of blood flow.
In fetal anemia, the blood becomes thinner and more diluted and so flows faster around the body. The blood is also lower in oxygen. This increased cardiac activity puts a strain on the fetus’ heart. Depending on the speed of the blood flow, the MCA Doppler can detect moderate to severe anemia. Immediate treatment by intrauterine blood transfusion is required to prevent severe complications such as hydrops fetalis, cardiac arrest or fetal death.
How is it used in HDFN?
As a result of standard prenatal testing, pregnancies at risk of developing HDFN are identified at the first prenatal visit, at no later than 10 weeks of pregnancy. In subsequent pregnancies, the risk is known from the beginning.
In pregnancies at risk of HDFN, MCA Doppler is recommended after 20 weeks’ gestation if there is a history of previous HDFN pregnancies, when alloimmunization of the pregnant mother has been confirmed, if maternal antibodies are elevated or if hydrops has been detected on imaging.
It is also used following an intrauterine blood transfusion (IUT) to help predict the timing of subsequent transfusions.
What are the benefits?
The use of MCA Doppler in HDFN-affected pregnancies has replaced the use of invasive testing to determine fetal anemia, such as amniocentesis, which carries risks of leaking amniotic fluid, bleeding, infection and miscarriage.
What are the risks and limitations?
No risks to the fetus have been identified through the use of MCA Doppler during pregnancy. However, a rate of 12% of false positives, or cases in which anemia was detected when there wasn’t any, has been reported in pregnancies of less than 24 weeks’ gestation. This is thought to be due to fetal activity and breathing or maternal eating habits.
