Ultrasound uses high-frequency sound waves to create images of the inside of the body. The technology has been around since the late 1950s in the form of 2D (two-dimensional) scans, but it wasn’t until the late 1970s that it became more widely used to look at fetuses. As the technology has advanced, doctors have introduced more advanced forms of ultrasound – notably 3D and 4D (three-dimensional and four-dimensional) scans. If a 3D image can be better viewed, 4D ultrasounds can help you see how your baby moves over time – the fourth dimension.