# A doctor uses ultrasound to carry out foetal scanning. Explain how ultrasound waves are used to produce an image of a foetus inside the womb.

> OCR A-Level Physics A (H556) — 5.2 Circular motion and gravitational fields · Explain · 5 marks

> Foetal scanning is a non-invasive imaging technique used routinely during pregnancy. Ultrasound waves have a frequency higher than 20 kHz, above the upper limit of human hearing.

## Mark scheme (5 marks)

1. Ultrasound waves are emitted from a source/transmitter directed at the womb
2. When the ultrasound waves reach a boundary between two different media (e.g. between the foetus and surrounding fluid/tissue), they are partially reflected back
3. The remainder of the waves continue to pass through / are transmitted into the next medium
4. A receiver/detector next to the emitter detects the reflected waves and measures the time between emission and detection
5. The time measurements are used to determine the position/distance of boundaries, building up an image of the foetus

## Key terms

- [ultrasound waves](https://www.gradenine.co.uk/glossary/ultrasound-waves)
- [foetal scanning](https://www.gradenine.co.uk/glossary/foetal-scanning)

## Related

- [Revision notes for OCR A-Level Physics A (H556)](https://www.gradenine.co.uk/learn)
- [How to answer "Explain" questions](https://www.gradenine.co.uk/tools/command-word-cheatsheet)
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Source: [GradeNine](https://www.gradenine.co.uk/q/a-doctor-uses-ultrasound-to-carry-fe35fc11) · Published by Druglandscape Ltd.