Explain how ultrasound waves are used in foetal scanning to produce an image of a foetus inside a womb.
Written & reviewed by James Millett — Biology (Imperial College London), PGCE Science (University of Cambridge).
Ultrasound waves have a frequency higher than the upper limit of human hearing, which is 20 kHz. They are used in medicine to produce images of structures inside the body without using harmful radiation.
Model answer (5 marks)
1. A probe (transducer) emits high‑frequency sound waves into the womb.
2. The waves travel through the amniotic fluid and encounter interfaces between tissues (e.g. foetal skin, organs).
3. At each interface part of the wave is reflected back to the probe.
4. The probe receives the echoes and measures the time delay between emission and return.
5. Using the speed of sound in tissue, the delay is converted to distance, and the data are compiled to construct a visual image of the foetus.
2. The waves travel through the amniotic fluid and encounter interfaces between tissues (e.g. foetal skin, organs).
3. At each interface part of the wave is reflected back to the probe.
4. The probe receives the echoes and measures the time delay between emission and return.
5. Using the speed of sound in tissue, the delay is converted to distance, and the data are compiled to construct a visual image of the foetus.
Examiner tips
- Use the word "emitted" for the probe, "reflected" for echoes, and "time delay" for distance calculation.
- Show the conversion: distance = (speed × time)/2.
- Mention that the image is built by scanning across the area.
Mark scheme (5 marks)
- Ultrasound waves are emitted/sent into the body (by a probe/transducer)
- The ultrasound waves are reflected at boundaries between different tissues/structures (e.g. the foetus and surrounding fluid)
- The reflected waves are detected (by the same probe)
- The time taken for the waves to return is used to calculate the distance to the reflecting boundary / the position of the foetus
- This information is processed to build up an image of the foetus
Key terms in this question
ultrasound waves · foetal scanning
Related
- All WJEC A-Level Physics (Wales) revision notes →
- How to answer a "Explain" question →
- Decode the mark scheme abbreviations →
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