University of Virginia
Physics Department

Fetal Ultrasound

A Physical Science Activity

2003 Virginia SOLs



Students will


Background Information

Ultrasound is a technique that uses sound waves to "look" inside a human body. Sound waves are bounced into your body and the reflections of the waves are captured by a machine which transform them into an image that can be read. Ultrasound is commonly used on pregnant women to help determine the general health of the fetus. The sound waves hit the baby and are reflected back to a sensor that translates the reflections into a picture. Real-time scanners are used to produce a continuous picture of the moving fetus which is then depicted on a monitor screen. Very high frequency sound waves of between 3.5 to 7.0 megahertz (i.e. 3.5 to 7 million cycles per second) are generally used for this purpose. The sound waves are produced and received by a small "transducer" that resembles a soap bar connected by a cable to the machine. It is responsible for converting the returning sound waves into signals that the machine can interpret. The transducer also converts signals from the machine into pulses of sound.


Gel is commonly used when a person gets an ultrasound. The gel is used to "couple" skin with the ultrasound transducer. The sound frequencies that are used could not pass through an air-body interface. The gel acts as a bridge between the machine and the person getting the ultrasound.




Student Activity

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  1. Complete the following activity sheet.


Activity Sheet

To print out the Activity Sheet only, click here.


  1. How does an ultrasound use sound to make a picture?



  2. Why do you think doctors use ultrasounds instead of x-rays to take pictures of fetuses?



  3. Brainstorm different reasons for having a sonogram. What information could you get from the image?



  4. The following three images are sonograms at different stages of pregnancy. Put them in order from youngest fetus to oldest fetus.




  5. What parts of the body can you see in each of the sonograms? At what point do you think that you could determine the gender of the baby?



  6. The following image is a three-dimensional sonogram. Is it easier or harder to see the fetus in this sonogram? How old do you think that the fetus is, compared to the above pictures? Label the body parts that you can see.




  7. The following images are also three-dimensional sonograms. What interesting thing was discovered about this baby by using a sonogram?




  8. What can you determine about the pregnancy from the following sonogram?




Answers to Worksheet

  1. Ultrasound works similarly to sonar mapping. A sound wave is sent into the womb, where it bounces off the fetus and comes back. The amount of time that the sound takes to return and the manner in which the sound reflects off of the fetus are used to make the image.
  2. Doctors use ultrasounds because they are much safer than x-rays. X-rays can be harmful to humans, which is why people usually wear heavy aprons when getting x-rays of bones. Fetuses are very sensitive, so x-rays are even more dangerous to them. But sound waves are not very dangerous at all, and there are no known side effects of ultrasounds.
  3. Some uses of ultrasounds are:
    • Determine the gender of the baby
    • Check for the number of fetuses in the womb
    • Check the growth of the child to verify the due date
    • Diagnose certain birth defects
    • Help select the delivery method
    • Guide amniocentesis or chorionic villus sampling
  4. The order from youngest to oldest is: #1 (8 weeks), #3 (14 weeks), #2 (19 weeks).
  5. In the sonograms you can see the head, body (chest/stomach), arms, hands, legs, and feet. If you look closely, you can see facial features in picture #2. You can determine the gender of the baby during the second trimester (weeks 13 - 27), so picture #2 or later is a good estimate.
  6. It is much easier to see the fetus in the three-dimensional sonogram. This fetus is 10 weeks old, so it is in-between image #1 and image #3.
  7. This baby has six toes!
  8. This is a sonogram of twins.



Look on the internet for information on ultrasounds and for more sonograms, and have the students analyze them. is a good site with samples from each trimester. The above pictures are from, which has a lot of information as well as images. Have the students bring in sonograms of themselves, if their parents kept them. Brainstorm other medical uses for sonar, which can also be found on the internet.


Students with Special Needs

Each student should be able to participate in this activity.

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Activity sheet to be completed during the laboratory.