Foetal genitalia. Coloured 3-D ultrasound scan of a male foetus. The foetus' penis and scrotum are seen at lower centre. 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.