Ultrasound uses high frequency sound waves. A transducer is placed onto the mother's abdomen, along with a gel that makes the sound waves more effective. The sound waves then reach the baby and are sent back to the transducer, creating an ultrasound image. In some cases, especially during very early pregnancy, an ultrasound is also performed trans-vaginally. While ultrasound is generally known as a safe practice, technicians are trained not to hold the transducer in one place for longer than a few seconds, because the baby can experience a rise in temperature. Some research shows that unborn babies generally react to ultrasounds in some way, sometimes by covering their eyes with their hands. Apparently, the practice is annoying to a fetus. As long as the ultrasound technician who is carrying out your scan for you is a qualified professional, and you only have one ultrasound or additional ultrasounds exclusively for medical reasons and not for "fun", ultrasound is still a tool that can provide valuable diagnostic information. However, I advise against visiting the "4-D ultrasound" establishments that have become popular in recent years, because they often employ less qualified staff.