Foetus with an oedema. Ultrasound scan of a foetus with a subcutaneous oedema. This buildup of fluid in the subcutaneous tissue (under the top skin layer) can cause miscarriages and is often a sign of underlying disorders such as hydrops fetalis (a type of prenatal heart failure). 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.