Understanding Your Ultrasound

Happy Couple with UltrasoundThose 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.)

The beauty of ultrasound is that, unlike slicing a loaf of bread, you can see all of these internal structures without actually cutting a slice of the mother! The sound waves and their returning echoes pass painlessly through skin, organs and other tissues. The 2D illustration (see p. 10) shows a 2D ultrasound image of a “slice” of a baby’s brain, as if you were looking down on it after the top of the head had been “removed.”

If the doctor wants to see a different organ, or the same organ from a different perspective, the transducer can be angled, moved to a different spot on the abdomen, or rotated. This is why ultrasound images may show slices of different body parts that are horizontal, vertical or in profile. The images on the screen are also refreshed approximately 30 times a second, so you can see the baby actually moving in real time on video.

Three-Dimensional (3D) Ultrasound

By adding the third dimension – depth – to an ultrasound image, we use the same sound wave energy, but the computer displays the returning echoes in a different way. Three-dimensional (3D) ultrasound is often used to show the outer surface of the baby, and produces images that are most recognizable to the parents. It may supplement the 2D imaging to make sure the baby is developing normally.

Three-dimensional ultrasound technology allows you to see the baby as if you were actually inside the uterus looking at the baby as it floats in the amniotic fluid. When used in this way, 3D ultrasound ignores everything between the surface of the mother’s abdomen and the baby itself. Unlike 2D ultrasound, however, you would not see the baby’s internal organs. The illustration (see p. 12) shows the strikingly recognizable image of a baby’s face as seen with 3D ultrasound.

When the baby is very small, early in the pregnancy, the 3D image might show the entire body. But as it grows larger, the image reveals only limited areas of the body, and the doctor or sonographer will move or angle the transducer to show different parts or perspectives. Like 2D ultrasound, 3D ultrasound can also capture the baby’s movement.

Color Doppler Ultrasound

The 19th-century Austrian mathematician Christian Andreas Doppler (1803-1853) was the first to describe the Doppler effect, a phenomenon by which motion affects the frequency – or pitch – of sound waves. If you stand next to a railroad track and a train rushes by, the sound of the train whistle is higher pitched as it approaches you and lower in pitch as it moves away from you. This is the Doppler effect. Similarly, as blood flows through arteries and veins in the body, the movement of the blood cells changes the pitch (frequency) of the sound waves echoing back to the ultrasound transducer.

These changes in pitch allow us to measure the amount and rate of blood flowing in arteries and veins. For example, in the illustration (see p. 13) the color Doppler image is combined with a twodimensional image of the head to show blood flowing through major arteries in the brain. Images like this one allow the doctor to look at important blood vessels in the developing baby.

Spectral Doppler Ultrasound

Instead of showing the flow of blood by color on the ultrasound image, as with color Doppler, spectral Doppler ultrasound produces a graph that shows the velocity (speed) of blood flow in an artery or vein as it varies over time. Like other forms of ultrasound, this can be used to create images from within the bodies of both baby and mother.

The picture (above) shows a spectral Doppler image of blood flowing between the placenta and baby through the umbilical cord. The screen is split: the top half shows a 2D image of the umbilical cord, and the bottom half shows the speed of blood flow over time. The peaks along the graph occur when the baby’s heart contracts, squeezing out a burst of blood at a high rate of speed. When the baby’s heart relaxes between contractions, and the flow of blood becomes slower, the height of the Doppler curve goes down. The spectral Doppler image can be used to evaluate blood flow through any artery or vein.

Excerpted from Your Developing Baby, Conception to Birth: Witnessing the Miraculous 9-Month Journey (Harvard Medical School Guides, McGraw Hill, 2008).