This is a completely passive device, it’s just like putting a microphone up to the baby, there are no transmitted emissions. One of the most wonderful things to ever happen to a woman can be pregnancy, and everyone remembers when they visited the doctor and heard the fast little heartbeat the first time.
With this fetal Doppler, you can repeat that experience any time you want. Plug in any 3.5mm headphones or earphones to listen as the babies heart beats, and as a cool added bonus, record the audio directly to an audio recorder to share with others anytime. Link