Learning Resources

Is ultrasound safe for humans?

When the ultrasonic wave propagates in the biological system, the function and structure or state of the biological system can be change, which is the ultrasonic biological effect. The magnitude of the biological effects depends on the size of the ultrasound and the length of the examination.


I’ve been advised to have a series of ultrasounds. Is this safe?

You may be offered extra ultrasounds to monitor the growth and well-being of your baby. There are many reasons why you may be offered additional ultrasounds. For example, you may have had complications in a previous pregnancy, or have a medical condition such as diabetes or high blood pressure. Your care provider may want to check your amniotic fluid levels or to check the position of your baby. You will also be offered extra ultrasounds if you are expecting twins.

Ultrasounds have been used in pregnancy for more than 30 years, and nobody has ever found them to be harmful. Ultrasounds are only recommended when medically necessary and there are clear guidelines on the use of them in pregnancy.


Will the way ultrasound works affect my baby?

During any ultrasound, some heat is generated and is absorbed by the part of the body being scanned. The scans you have while you're pregnant produce very little heat (less than one degree centigrade). They are therefore fine for you and your baby. It is only if the temperature of the scanned body tissue rises by four degrees, say, from 36 degrees Celsius to 40 degrees Celsius, that it could be harmful.

It might also reassure you to know that the type of routine scanning used to get two-dimensional images of your baby uses a low intensity of ultrasound, and is spread over a large area. Your baby is moving, and the fluid he is in helps to spread any heat.


What about other types of technology for monitoring my baby’s well-being?

Other types of scan, such as Doppler and colour scans, work by concentrating a beam of sound in a small area. Although this can cause more heat, it would have to be held in the same place for a long time to raise the temperature much. Doppler is used to look at blood flow, and as blood is constantly moving, this distributes any heat produced.

Some scan machines automatically reduce the power of the ultrasound beam when Doppler is used, to reduce the intensity of the beam.

Hand-held Doppler machines and cardiotocographs (CTGs), which are used to listen to your baby’s heartbeat, also use ultrasound, but intensities are low and they are considered safe.

Vaginal ultrasounds may heat a little more quickly than scans through your tummy, as the vaginal probe is warmed by your body, but again, the probe would have to be left inside for a long time before there was any real rise in temperature.


How does the sonographer know how much heat the ultrasound is producing?

Ultrasound machines have what’s known as a thermal index displayed on the screen. This gives a rough guide to the amount of heat that might be produced after long exposure.

Most ultrasounds have a very low thermal index and can be used without any time limit. Doppler and colour scans have a slightly higher thermal index and scans should be restricted to 30 minutes. In most cases, getting a Doppler reading should only take a few minutes.


How can I be sure the ultrasound is being performed safely?

The guidelines say that ultrasounds should only be done by fully trained staff who know how to perform the scan safely. Trained staff know to perform ultrasounds that are medically necessary only. They also know to use low levels of ultrasound wherever possible, and to do the scan as quickly as they can. They know that:

A thermal index (heat index) for soft tissue should be used for a fetus under 10 weeks.
A thermal index for bone should be used after your baby is 10 weeks old.

Most doctors and scientists agree that it's extremely unlikely that ultrasounds can cause any harm, and that the benefits from serial ultrasounds in monitoring pregnancies outweigh any potential risks.


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