The Science of Ultrasound in Pregnancy

Seeing a positive result on a pregnancy test for the first time was a moment I’ll always remember. Like other mums-to-be I’m sure, I began wondering whether my baby was healthy, and started looking forward to having a scan so I could see my developing baby.

Showing the portable ultrasound and transducer to the audience CC-BY-science made simple

Showing the portable ultrasound and transducer to the audience CC-BY-science made simple

However, unlike most other mums-to-be, I regularly use an ultrasound machine at work. In science made simple‘s ‘Engineering for Life: From Cradle to Grave‘ KS3 show, I choose a volunteer from the audience and use the ultrasound to look inside their neck. The whole audience is able to see the major structures inside the volunteer’s neck, including seeing their carotid artery pulsating as their heart pumps blood around their body!

So of course, I was itching to use the ultrasound on myself!

But what actually is ultrasound, and why do we use ultrasound machines to look at developing babies?

What is ultrasound?

Ultrasound is simply sound that is too high-pitched for our ears to hear. Young, healthy human ears can hear up to about 20,000 Hz (caused by something vibrating 20,000 times in one second!) We tend to lose the ability to hear at these high frequencies as we age. I’m 27, and I can hear up to about 15,500 Hz. When I test my audience’s hearing ability in our ‘Music to your Ears‘ show, young teenagers can hear the very high pitched noises long after I cease hearing anything! You can find out the highest frequency you can hear, here.

Anything above the audible human limit can be considered ultrasound. Many species of bats use ultrasound in a process called echolocation – the biological equivalent of sonar! They rely on their ears to ‘see’ the world around them, rather than their eyes. They produce a high pitched noise inaudible to humans, which travels through the air and bounces off objects (echos). How long this echo takes to get back to them, and the intensity of the sound wave, provides them with information like how far away the object is, and where it is.

Because ultrasound is normal sound (that humans can’t hear) it is considered completely safe to use ultrasound to image an unborn child.

Medical ultrasound


CC- BY-SA BruceBlaus own work

Ultrasound machines typically use frequencies in the region of 1-20 MHz. In a similar way to bats creating an image of their surroundings using echos, ultrasound machines use echos to create an image of the inside of the body. Because sound travels differently through different substances, ultrasound machines can differentiate between tissue, bone and fluid. The higher the frequency used, the higher quality the image will be. However, higher pitched sound won’t penetrate as far, so lower frequencies are needed to image deeper structures in the body.

What happens during an ultrasound scan?

The sonographer (a person trained to use an ultrasound machine) will place a handheld device called a transducer against your skin. This transmits high frequency sound through your body, which will echo off the baby and generate an image.

For a clearer image, pregnant women are advised to have a full bladder before the scan. Having a full bladder pushes the uterus slightly higher, allowing for a better view of the baby. Having the scan shouldn’t hurt at all, but some women report it feeling a little discomfort. I only felt a little discomfort during my first scan, as the transducer pressed down on my full bladder!

The sonographer will also need to put a small amount of gel onto the transducer. The gel creates a bond between the transducer and your skin, eliminating any air between the two which would result in a lower quality image, and allowing the signal to be transmitted straight into the tissues.

Baby Gamble at 13 Weeks. All rights reserved

This is my first ultrasound picture, and the baby was dated to be 13 weeks old. It was a relief to be told there was just the one baby!

Why can’t I have an x-ray to see my baby?

Having an x-ray on an area of your body carries only a very small risk when done on an infrequent basis, and is not associated with increased risk of miscarriage or birth defects if done whilst pregnant.

However, having an x-ray does expose the area to short bursts of radiation, which can increase the risk of cancer. The risk of this is still very low, but as an unborn baby’s cells are rapidly dividing they may be more susceptible to damage, and healthcare providers would only offer an abdominal x-ray during pregnancy in extreme cases.

Ultrasound scans during pregnancy – what are they for?

Unless your healthcare provider has recommended you are seen more regularly, you will often be offered just two scans during pregnancy. Once at 12 weeks, and another at 20 weeks.

At 12 weeks, you can see the baby’s heartbeat, and take some measurements to estimate the baby’s age, and, identify if you are a high or low risk for having a baby with Down’s Syndrome and possibly other conditions. This will normally only be done with your permission, and is not a definite answer, only an indication of the risk factor.

Down’s Syndrome, or Trisomy 21, is where the baby develops with an extra copy of chromosome 21. Healthy humans should have 2 copies of each chromosome – one from each parent. The cell division process however, is not always perfect, occasionally resulting in an abnormal chromosome number. For most other chromosomes, inheriting an extra copy will not result in a viable foetus and the pregnancy will not continue. During the 12 week scan, if requested, the sonographer will measure the amount of fluid at the back of the baby’s neck, also called nuchal translucency. All babies have fluid here, but an increased amount of fluid is an indicator of an increased risk of Down’s Syndrome. If requested, further screening can then be done for a more definitive answer.

At 20 weeks, the baby’s gender can be identified. Tell your healthcare provider if you do not wish to know the baby’s gender at this point!

My 20 week scan is on Friday – can’t wait to find out if we’re having a son or a daughter!

UPDATE: On 20th June 2017, our beautiful baby boy was welcomed into the world!

Baby Gamble, a few minutes old. All rights reserved.

ZG headshot1.jpg

About the Blogger – Zoë Gamble

Zoë is a STEM Communicator and trainer for science made simple. Her background is in biomedical science, and she has over 6 years’ experience presenting science to audiences around the world.


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