An ultrasound is a diagnostic test that has many uses, from diagnosing serious illnesses to determining a baby's sex. Ultrasounds are often used on the skin's surface, but sometimes the tool is inserted through a natural opening in the body. For example, an ultrasound tool may be inserted into the esophagus to take pictures of the heart. Ultrasounds can also be used on the cranium and bones, and can be done in 3D or 4D. Follow the links below to find WebMD's comprehensive coverage about how ultrasounds are performed, why they are used, what to expect during an ultrasound, and much more.