Medical professionals have used Doppler-based ultrasound for many years, not only for pregnant women but for medical and surgical applications. The Doppler Effect is based on the idea that you can locate things in space by sending out a frequency pulse train, which bounces off an object and returns to you at a slightly different frequency. Measuring the returning frequency, you can determine where things are inside the body.
Ships use a similar sound-based technology, called sonar, to locate underwater ridges as well as to track other ships. A ship sends out a tone, which bounces off other objects and returns as a different frequency. The ultrasound test performed by a fetal Doppler works in much the same way: the sound bounces off your unborn baby, generating a black and white image that's displayed on a computer screen.
Fetal dopplers can be quite expensive but there is an affordable bonding opportunity for you and your baby. If your pregnancy is at 20 weeks or more, you can start to hear you baby's heart beat with a baby heart rate monitor. Listening your baby's heartbeat can be an amazing experience.
Dopplers designed for private use are pocket-sized, battery-operated devices. They usually come with a handset of controls, a built-in speaker and a transducer which is placed against your bump.
There is no scientific research available to suggest that ultrasound is unsafe. In 2002, the British Medical Ultrasound Society issued a statement confirming that there has never been any evidence of harm resulting from the use of ultrasound dopplers in pregnancy. However it's important to remember that this research was based on the work of health professionals, who have been trained in its use under strict guidelines.
While there is no evidence as yet, that ultrasound is harmful in any way, there is also no research available to show the effects of a mother using a Doppler to listen to her baby two or three times a day throughout pregnancy. If you do decide to buy one, you may like to be cautious about how often you use it.