For years, physicians have relied on traditional ultrasound tests to learn how far along a woman is in her pregnancy, as well as the size, health, and position of the developing baby. Now, four-dimensional (4D) ultrasound lets women and doctors look at facial features and watch the growing baby move.

An ultrasound scanner (or transducer) sends sound waves through the body. These sound waves bounce back to the scanner and produce an image on a computer screen. Ultrasound is generally safe for the developing baby and mother. A technician applies a gel before placing and moving the transducer over the skin. The transducer may also be inserted into the vagina.

The conventional mode of ultrasound scanning is two-dimensional (2D). In other words, the image is made up of thin slices, and only one slice can be seen at a time. Although such an image is very informative to ultrasound professionals, to the average person, the picture may not look at all like a baby.

With 3D ultrasound, a volume of echoes is taken, stored digitally, and shaded to produce life-like images of the fetus. A 4D ultrasound takes the images produced by 3D ultrasound and adds the element of movement. Now, the life-like pictures can move and the activity of the fetus can be studied.

The ability to obtain clear images and activity will depend on the stage of pregnancy and the position of the fetus during the ultrasound exam.