Foetus. Coloured 3-D ultrasound scan of a foetus. The arms (upper left) and legs (lower centre) can be seen, along with the umbilical cord that runs from the foetus to the placenta (right). This was taken in the second 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.