You see a small rhythmic flicker of movement on the screen. You hear a pulsing sound from the ultrasound machine. What is it? Is it actually the sound of a heartbeat? Or is it just a “manufactured sound“? You may have heard the term doppler referred to during an ultrasound, but what does that even mean?
Have you ever been standing on a street corner when a motorcycle passes? The zooming noise from the engine seems to increase in pitch/frequency as the motorcycle gets closer to you and then decrease in frequency as the motorcycle drives away from you. That change in frequency is called a “Doppler shift”.
Did you know that ultrasound doppler works the same way? It measures the sound waves that are reflected from moving objects like red blood cells (aka “the motorcycle”).
When the walls of the fetal heart are moving and pumping blood, the ultrasound machine is sending and receiving sound waves that are focused on the heart. It is receiving signals of different frequencies based on the motion of the blood flow.
The sound you’re hearing from the ultrasound machine is the change of pitch from different frequencies of the returning sound waves based on the speed and direction of the blood flow in the heart. It is the same sound you would hear if you were listening to an adult heart.
While the embryonic heart actually begins to beat between 5-5 1/2 weeks (based on the gestational age calculated from the beginning of your last menstrual period), the ultrasound machine is only able to detect it at approximately 6 weeks. The presence of a fetal heartbeat is just one of the several factors we use at Focus Women’s Center to determine if your pregnancy is viable or not.
*Edelman, Sidney. Understanding Ultrasound Physics, 4th edition. Woodlands, Texas, ESP Inc., 2012.
*Gagen-Ansert, Sandra. Textbook of Diagnostic Sonography, Volume Two. Elsevier Mosby, 2012.