Ultrasound imaging is based on the same principles involved in the sonar used by bats, ships at sea, and anglers with fish detectors. As a controlled sound bounces against objects, its reflected waves can be used to identify how far away the object is, how large it is, its shape and its internal consistency (fluid, solid or mixed).
The ultrasound transducer functions as both a loudspeaker (to create the sounds) and a microphone (to record them). For obstetric ultrasound, when the transducer is pressed against the skin, it directs a stream of inaudible, high-frequency sound waves into the lower abdomen and pelvis. As the sound waves echo from the embryo or foetus and surrounding structures in the uterus, the sensitive microphone in the transducer records tiny changes in the sound's pitch and direction. These signature waves are instantly measured and displayed by a computer, which in turn creates a real-time picture on the monitor. The live images of the examination can be recorded on videotape. In addition, still frames of the moving picture are usually "frozen" to capture a series of images. Conventional ultrasound displays the images as thin sections (like looking at single slices of bread in a loaf). 3-D ultrasound is the result of modern computer technology, that can reformat data into three-dimensional images (like looking at the entire loaf of bread from various projections). 4-D ultrasound is 3-D ultrasound in motion.
Doppler ultrasonography is the application of diagnostic ultrasound to detect moving blood cells and measure their direction and speed of movement. The Doppler effect is used to evaluate blood flow by measuring changes in the frequency of the echoes reflected from blood cells.
The movement of the embryo or foetus and the heart beat can be seen as an ongoing ultrasound "movie." Most ultrasound devices also have an audio component that processes the echoes produced by blood flowing through the foetal heart, blood vessels and umbilical cord. This sound can be made audible to human ears and has been described by patients as a "whooshing noise."