A medical team uses ultrasound waves to produce an image of a foetus inside a womb. Explain how ultrasound waves are used to produce this image, and why ultrasound is chosen for this purpose rather than X-rays.

Ultrasound waves are emitted by a transducer and travel through the mother’s body. When they reach a boundary where the acoustic impedance changes – for example between fluid and tissue or between different tissues – part of the wave is reflected back to the transducer while the rest is transmitted into the next medium.

The transducer also acts as a receiver. Because the speed of sound in soft tissue is essentially constant, the time taken for a reflected pulse to return is proportional to twice the distance to the reflecting surface. By measuring this time and multiplying by the speed of sound, the distance to each boundary can be calculated.

Multiple reflections from many different boundaries are recorded, and the transducer’s electronics convert the set of distances into a two‑ or three‑dimensional image of the foetus.

Ultrasound is preferred to X‑rays because X‑ray photons can ionise cells, potentially causing mutations and harm to the developing foetus. Ultrasound uses mechanical waves, which are non‑ionising and safe for the fetus and mother.