A particular enzyme test is usually done only when the fetus is at risk for the related disease, for example, if the disease runs in the family. Another common prenatal test is ultrasound imaging . Ultrasound uses sound waves to create an image of the baby inside the mother. Many physical problems can be detected with ultrasound. If something does not look right, the doctor may recommend more tests.
Doctors remove some cells surrounding the fetus.
These cells are treated with a special dye and photographed through a microscope. In the photograph, the chromosomes will look striped, because of the dye. Corresponding chromosomes will have the same pattern of stripes and be the same size and shape. A special technique is used to rearrange the chromosomes into pairs. The picture that is created is called a karyotype .