Foetus' face. Coloured 3-D ultrasound scan of a 25 week old foetus' face. The foetus' nose is at lower centre right, the mouth is below it. At this stage the foetus measures around 22 centimetres in length. It may respond to loud noises. Ultrasound scanning is a diagnostic technique that sends high-frequency sound waves into the body via a transducer. The returning echoes are recorded and used to build an image of an internal structure. Foetal ultrasound scanning is routine during pregnancy. 3-D scanning uses computer technology to return more detailed images than conventional 2-D scans.