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Ultrasound is one of the most useful procedures for monitoring a baby's prenatal development. With ultrasound, doctors can check for defects of the head, spine, chest, and limbs; diagnose serious conditions like placenta previa or breech birth; and check to see whether the mother will have twins or triplets.

Ultrasound can be used anytime during pregnancy from the fifth week until delivery. It uses inaudible sound waves to "see" the baby inside the uterus. These sound waves bounce off solid structures in the body and are transformed into an image on a screen.

Here's how ultrasound works. Pretend this tennis ball is an organ in the body. This piece of glass represents the ultrasound image. Like this piece of glass, an ultrasound image is actually flat and two-dimensional.

If we could pass this tennis ball through the glass, the ultrasound image would show wherever the two are in contact. Let's watch the same thing on an ultrasound.

The white ring is the reflected image of the outer part of the tennis ball. Like many organs in the body, the tennis ball is solid on the outside, and hollow on the inside. Solid structures, like bones and muscles, reflect sound waves that show up as light gray or white images.

Soft or hollow areas like the chambers of the heart don't reflect sound waves. So they show up as dark or black areas.

In an actual ultrasound of a baby in the uterus, the solid structures in the baby’s body are transmitted back to the monitor as white or gray images. As the baby moves back and forth, the monitor shows the outline of his head. The eyes show as dark spots in the head. The region of the brain and the heart are also shown.

Remember, ultrasound only shows a flat image of the baby. A superimposed illustration of the fetus shows how the fetus actually looks in the uterus.

Ultrasound is still one of the best methods for physicians to visually diagnose major physical defects in the growing baby.

Even though there are no known risks for ultrasound at present, it is highly recommended that pregnant women consult their physician before undergoing this procedure.

Review Date 1/10/2022

Updated by: John D. Jacobson, MD, Department of Obstetrics and Gynecology, Loma Linda University School of Medicine, Loma Linda, CA. Also reviewed by David Zieve, MD, MHA, Medical Director, Brenda Conaway, Editorial Director, and the A.D.A.M. Editorial team.