Traditionally, 2D ultrasound allows you to see the fetus in cross-section only. For example, when looking at the fetus's face, 2D ultrasound will allow you to see the profile, but not the entire face in one picture. 3D ultrasound, on the other hand, allows you to see the surface of the whole face in one picture — much like taking a photograph. 4D ultrasound adds the dimension of time, so instead of seeing a 3D snapshot of the face, you get the face moving in real-time (grimacing, opening and closing its eyes, sticking out its tongue), just as in a videotape.
The medical reasons for using 3D and 4D ultrasounds are limited. Currently, we use them when a fetal abnormality is seen or suspected. 3D and 4D ultrasounds can help doctors determine the extent of the abnormality, and parents better understand it.
When used for medical purposes such as screening for abnormalities or tracking a known fetal abnormality, 3D and 4D ultrasounds appear to be safe. Although several studies of ultrasound in animals have shown effects on brain cells, hundreds of others have concluded ultrasound in pregnancy to be safe. To minimize risk, always have your ultrasound in a center with experienced technicians or physicians.
Recently, shopping malls and other freestanding commercial sites have started offering pictures or entertainment videos of fetuses. Remember — ultrasound is intended for medical diagnosis, and "entertainment" ultrasound is considered an unapproved use of a medical device by the FDA. These sites might not be equipped to answer questions that arise during your visit ("Does everything look normal?"), abnormalities could be missed, and you may gain false reassurance. The American College of Obstetricians and Gynecologists and the American Institute of Ultrasound in Medicine, the main overseeing bodies for ultrasound in obstetrics, discourage the use of ultrasound for non-medical purposes.