Foetus's face. Coloured 3-D ultrasound scan of a foetus' face. The foetus' nose is at centre left, the mouth is below it. 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.