Doctors use ultrasound waves to carry out foetal scanning. Explain how ultrasound waves are used to produce an image of a foetus inside the womb.

Ultrasound scanning uses high‑frequency sound waves that are emitted from a probe placed on the mother’s abdomen. The waves travel through the body at a speed of about 1540 m s⁻¹. When they reach a boundary between tissues of different acoustic impedance – for example, between fluid and foetal tissue – part of the wave is reflected back to the probe. The probe detects these reflected waves and measures the time taken for the echo to return. Because the speed of sound in tissue is known, the distance to the reflecting surface can be calculated from the travel time. By emitting many pulses in rapid succession and recording the echoes from different angles, a computer assembles the distance data into a two‑dimensional image of the foetus.

Key points: 1) Ultrasound waves are emitted from the scanner into the body. 2) They have a frequency above 20 kHz, outside human hearing. 3) Reflections occur at tissue boundaries or the foetus surface. 4) The scanner detects the reflected waves. 5) The return time is used to calculate distance, allowing a computer to build an image.