MODEL RELEASED. Ultrasound scan. Health worker applying gel to the abdomen of a pregnant woman about to have an ultrasound scan. Ultrasound is a diagnostic technique in which very high frequency sound waves are passed into the body through a transducer. The reflected echoes are detected and used to build a picture (upper right, on monitor) of a foetus in the uterus or of other internal structures. Applying gel to the skin ensures that there is good contact with the transducer. Ultrasound scanning is often performed during pregnancy as a routine check-up but can be performed at any time when there is a need to view the foetus.