An echo device emits ultrasounds. For instance, a number of waves per second or per minute. These ultrasound waves are not rays, as is the case in an X-ray device.
The sound waves are sent to a specific part of the body, for instance to the heart, or in the case of a pregnant woman, to the foetus. Human tissue bounces the waves back to the echo device (the transducer). With the use of these sound waves, images or movies are made of the heart or of a foetus. The images can be two, three or four dimensional and form the final result for an ultrasound image. In case of the Babywatcher, the images will be 2D.