Foetus's face. Coloured 3-D ultrasound scan of a foetus holding its hands (lower right and lower left) over its face. The foetus' nose is at lower centre, with the mouth below that. This was taken in the third trimester of pregnancy. 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.