": The astonishing medical machine resting on this pregnant woman's abdomen in a Philadelphia hospital is "looking" at her unborn child in precisely the same way a Navy surface ship homes in on enemy submarines. Using the sonar principle, it is bombarding her with a beam of ultra-high-frequency sound waves that are inaudible to the human ear. Back come the echoes, bouncing off the baby's head, to show up as a visual image on a viewing screen."