Ultrasound (or “sonogram”) is a test that uses sound waves reflected from the baby to create a picture of the baby on a special computer screen. While ultrasound does not provide a crystal clear picture of the baby, by 16-18 weeks of pregnancy it does enable a doctor, skilled in interpreting sonographic images, to identify some physical types of birth defects such as limb abnormalities, heart defects, and some abnormalities of the brain and spine. Ultrasound is mostly limited to identifying problems that are of a physical or structural nature. Ultrasound cannot detect problems of function such as mental retardation, blindness, or deafness, nor can it detect chromosome abnormalities such as Down syndrome. In some cases, ultrasound findings may lead a doctor to suspect a chromosome abnormality and therefore offer further diagnostic testing.