Handheld fetal dopplers are actually quite easy to use and understand. A fetal doppler is used to listen to a baby’s heartbeat while it is still in the womb. By using ultrasound technology, a fetal doppler heartbeat monitor sends high frequency sound waves into your abdomen. When the sound waves encounter movement, such as your baby’s heartbeat, it sends a signal back to the device through the handheld probe. The signal is then translated into a sound which is then amplified so you can hear it.