Ultrasound: what it is, how it works and impact of exposure

Published 1 February 2012

1. What is ultrasound?

Ultrasound is defined as high frequency sound waves with frequencies above 20 kilohertz.

It is useful as a diagnostic tool in pregnancy because it can provide detailed information about the health of an unborn child without exposing the mother or child to ionising radiation such as x-rays.

2. How does it work?

Ultrasound imaging uses the echoes formed when the sound waves bounce off body organs to build up a picture by analysis of the amplitude, arrival times and sometimes the frequency of echoes returned by different tissues. Highly reflecting structures such as gas and bone return high amplitude echoes which are shown as bright spots on the ultrasound scan.

The echo arrival time gives information about the position of the structure and the deeper it is, the longer it will take the echo to come back to its source. Using this technique a single image can be built up from the echoes returned from a region of interest.

3. Are there any risks from ultrasound scans?

At present there is very little scientific information available with which to assess the impact of exposure to ultrasound, particularly on the unborn child. However, antenatal ultrasound scans have been used for many years without apparent ill-effect.

Subtle effects have been reported in studies of brain development in small animals, and some studies in humans indicate changes in neurological functions following in utero exposures.

While these data are not considered to provide clear evidence of a specific hazard, the possibility of subtle long-term effects cannot be ruled out. As well as this, the evidence on the effects of fetal ultrasound in humans mainly date from some time ago, and used different techniques and lower exposures than are used today. There is little direct evidence on the safety of modern techniques, but no ill effects have been reported.

Health Protection Agency (HPA) advises that people should not hesitate to continue using ultrasound for diagnostic and other medical purposes, including in pregnancy. Such use has an established track record of safety and is regulated.

4. Souvenir scanning

In recent years, ultrasound scans without any diagnostic purpose have become increasingly popular with parents-to-be.

These have become known as ‘souvenir scans’ as they provide the parents-to-be with images or recordings of the unborn baby as keepsakes - however many of these scans offer no clinical benefit.

5. HPA advice

There would appear to be no reasons to withhold diagnostic imaging during pregnancy at present. Therefore HPA advises that people should not hesitate to continue using ultrasound for diagnostic and other medical purposes, including in pregnancy. Compliance with existing exposure guidelines should ensure that no adverse acute effects can occur.

The HPA’s Advisory Group on Non-Ionising Radiation has recently published a report on the use of ultrasound, and the Agency has taken note of its recommendations on the topic.

The HPA advise that although there is no clear evidence that ‘souvenir scans’ are harmful to the fetus, parents-to-be must decide for themselves if they wish to have souvenir scans and balance the benefits against the possibility of unconfirmed risks to the unborn child.

Further, HPA recommends that ultrasound examinations should always be performed by specialist healthcare professionals, who are well trained in ultrasound safety, and who can ensure compliance with exposure guidelines. Such specialists can also offer the appropriate advice and counselling should any abnormality or adverse event be observed. These issues may also inform any decision about souvenir scans.

6. Future research

Research into the safety of ultrasound continues to be carried out both in the UK and abroad. Because of the uncertainties surrounding the possibility of neurological effects from prenatal exposure, the HPA supports the need for such research, particularly with regard to in utero exposures.

The Agency is continuing to monitor worldwide research in this area and will update its advice based on the careful analysis of the latest evidence.