Posted on January 2nd, 2015 by

How Ultrasound Works

Let’s explain ultrasound, how ultrasound works, and why we call this technology ultrasound!

What is Ultrasound?

Ultrasound is just that…ULTRAsound, or sound waves that operate at a frequency far beyond human hearing.

*Human hearing ranges from about 20Hz (Hertz) to 20,000Hz (or 20kHz)*
*Ultrasound operates in the megahertz (MHz) or millions of Hertz!*

Fetal ultrasound probes operate at a frequency range of about 2-13MHz. Also, the use of Doppler technology is another form of ultrasound. Your healthcare provider may use a hand-held Doppler transducer to detect your baby’s heartbeat starting at about 10 Weeks. The frequency of these dopplers range from 2-5MHz. These dopplers require special training. Only those educated on how to distinguish maternal blood flow from fetal heart tones should use them.

Additionally, Doppler technology can also play a role in conjunction with an ultrasound machine, but only formally-educated sonographers should utilize it. This feature emits a higher energy, therefore, power settings should be set very low. An example of this type of Doppler is when your sonographer is scanning your baby and uses doppler to hear the heartbeat while it’s also shown on a graph below the image. We like to use this very sparingly early in the first trimester!

Most importantly, Ultrasound is not radiation!

X-Ray is ionizing radiation; ultrasound is like sonar or a fish-finder!

How Ultrasound Works

Sound waves penetrate skin and the tissues below it with the help of gel which aids in conducting the waves. Remember the goopy stuff we squirt on your belly? Once the sound waves hit the target of interest, they bounce back and result as an image on the monitor. It’s up to the operator to efficiently and properly adjust many settings to optimize for the best image. Credentialed sonographers know how to do this best! After all, Knobs and Buttons 101 is a large part of our training and education:)

The image produced is a 2D image (the gray-scale image) which allows us to see in two planes only at one time. An example of a 2D image is seen below. Baby is shown upside down here!

9 Weeks pregnant, ultrasound, 9 Week Embryo

9 Week Embryo in 2D

Because a 3D image is made when the sonographer makes a sweeping motion with the probe, several 2D images are saved all at once. We then have a “box” of information to work with that allows us to see the outside of Baby instead of “through” Baby. Below is a 3D image at 9 Weeks:

how ultrasound works

9 Week Embryo in 3D

Hopefully, this post helps you understand ultrasound a little better when you go for your next scan. I wish you all uneventful pregnancies and happy, healthy babies!

I hope you keep coming back to have all your ultrasound questions answered, so email me at!

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