The technical term for a specialized sonogram of a baby’s heart before they are born is a fetal echocardiogram (fetal echo). This type of examination is usually performed for the first time at 20-24 weeks gestation. As is true with any test, before you have a fetal echo it is important to understand what it can and cannot diagnose.
A fetal echo is designed to gain a general understanding of the structure of the heart-how blood flows through the heart (see Normal Blood Flow). The so called “plumbing” of the heart is important to examine. We look to ensure all the blood vessels are attached/connected to the heart in a normal way.
In addition to examining the heart connections, we examine the heart valves. There are 4 heart valves designed to keep blood moving forward. They are meant to be “one way” valves. We look to make sure the valves are the correct size for gestational age, open easily with blockage, and do not allow back-flow/regurgitation.
A fetal echo also looks at overall cardiac performance. We look to ensure the heart is squeezing well-pumping blood efficiently through the circulation. We examine the baby to make sure there is not back up of fluid because of poor heart pumping. Likewise we examine the heart rate to make sure the heart is beating in a coordinated way. At times the heart can have an electrical “short circuit” problem called an arrhythmia. This can also be identified on a fetal echo.
Finally large holes or connections can be noted on a fetal echo. These large holes allow blood to flow inappropriately between heart chambers. Typically these holes occur in one of the heart walls call a septum. There are 2 septa that partition the left side of the heart from the right side. A hole can exist in the top septum (an atrial septal defect or ASD) or it can exist in the bottom septum (a ventricular septal defect or VSD).
As you can tell much information is gathered on a routine fetal echo. There is some information that even a fetal echo cannot determine. One such limitation is detecting small holes or connections in the heart. These small connections can be hard to diagnose because the small size and small amount of blood flowing across these holes. All babies have a hole between the top chambers in the womb. This “normal” hole is called a PFO. It can be hard to distinguish a small ASD (described above) from the normal PFO as they can be similar in size and in location.
Little blood flows to the baby’s lungs in the womb because the lungs are filled with fluid and do not provide oxygen to the blood stream. Because only a small amount of blood flows to the lungs only a small amount returns from the lungs to the heart. The small vessels that take blood from the lungs to the heart are called pulmonary veins. Minor abnormalities in the pulmonary veins can be missed by a fetal echo because of the small size of the vessels and small amount of blood flowing through these vessels.
The last major area that can be a challenge to fully evaluate on a fetal echo is the aortic arch. The aortic arch is the main blood vessel that takes blood from the heart to the body. Major abnormalities or obstruction in this vessel are often apparent on a fetal echo, but more minor problems can be hard to diagnose. The challenge here is that a normal connection found in all baby’s hearts that allows blood to bypass the lungs for the placenta called the ductus arteriosus can at times mask the obstruction in the aortic arch.
A thorough fetal echo is a powerful tool. While some small or minor abnormalities can be hard to diagnose, most major structural, electrical, or performance related issues can be diagnosed. Being prepared before this test helps to build in expectations and plan for any additional testing needing following delivery.
Michael Day, M.D.
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