If you are expecting, you may have recently purchased a baby heart doppler unit to hear your baby's heartbeat.
These unit rely on what is known as the "doppler effect" to allow you to hear your baby's heart clearly and strongly.
The doppler effect is found in both light and sound waves.
If you took an astronomy course in high school or college, you may have learned about using the doppler effect with light to ascertain whether a star is moving towards or away from us.
With sound, the effect works in a similar manner but with the pitch of sound waves.
Think of how the sound of a train's horn changes pitch as the train comes near you and then away again.
This is not because the sound of the horn is actually changing, but is the result of the doppler effect on the pitch of the sound.
Baby heartbeat monitors use the doppler effect to allow you to hear the sound of your baby's heart.
In order to use a baby heart monitor, you will need to have some ultrasound on your belly to help with the sound transfer.
To use a baby heart doppler device properly, you will place the gel on your stomach and then slide a probe on top of the gel until you are able to hear your baby's heartbeat.
A baby's heart rate is much more rapid than an adult's heartbeat, ranging from about 120-160 beats per minute.
You should be able to use most baby heart monitors from twelve weeks into your pregnancy, and some units may work as early as ten weeks.