Understanding Your Ultrasound

Those First ‘Baby Pictures’ Are Also a First Look at Baby’s Health

By Peter M. Doubilet, M.D., Ph.D., and Carol B. Benson, M.D.

Editor’s Note: What expectant parent doesn’t look forward with glee to that first ultrasound, a chance to finally peek inside the bump and see what’s cooking? But an ultrasound exam is so much more important – and so much more revealing – than just a set of “baby pictures.” This excerpt from Your Developing Baby, Conception to Birth will give you the lowdown on how the technology works, and just how much it can tell us.

An ultrasound transducer both looks and functions like a large two-way microphone: it transmits inaudible, highfrequency sound waves into the body and then “listens” for the echoes that come back as the sound waves bounce off internal structures of mother and baby. A computer in the ultrasound machine then translates these reflected sound waves into images that you can see on a screen.

The images can be of various types, and a single sonogram often produces more than one type of image.

Two-Dimensional (2D) Ultrasound

Two-dimensional refers to a flat surface, so 2D ultrasound produces images that are like slices through the body. This is the most commonly used form of ultrasound because it is best suited to examining the baby’s internal organs to make sure development is normal.

A good way to think of 2D ultrasound is by imagining a loaf of bread. When you slice a piece, you can look at the flat surface of the slice (a two-dimensional object), and see the texture of the dough, as well as the location and size of any air pockets, nuts or raisins.

In a similar way, when the transducer is placed on the mother’s abdomen, it sends back information that is translated into an image “slice.” The sound waves pass through her skin, her abdominal muscles, the wall of the uterus, the amniotic fluid surrounding the baby, the baby’s skin, and into the baby’s internal organs – such as the heart or the brain. The information sent back to the computer creates a twodimensional slice of all of those tissues and structures, allowing the doctor to examine the baby’s organs closely. (CT scans use similar imaging techniques.)