How does a fetal Doppler work?
Whether clinical or at-home, a fetal Doppler machine is similar to an ultrasound machine in that it works through the use of sound waves. (However, unlike your standard ultrasound, a fetal Doppler does not produce an image of your baby).
The fetal Doppler is actually named after the Doppler effect, which explains how the echo of sound waves change based on their closeness to an object. By measuring this change, a fetal Doppler picks up the beats of your baby’s little heart.
Inside the machine and your belly, the process looks something like this:
Wave emitting – All fetal Doppler machines have a probe or transducer. When placed on the belly, this small part of the machine emits quick pulses of ultrasound sound waves into the body.
Wave reflection – Your baby’s little heart will “pick up” these sound waves, reflecting them back toward the machine. This echo action changes the frequency of the sound waves, making the start of an actual noise.
Wave receival – When the echoed waves reach the Doppler machine, it converts them into a sound—and voila! A speaker or connected earphones can play you the thump-thump of a tiny heart. Some machines may also display the BPM.