Karyotypes refer to the number and appearance of the chromosomes in an organism. It shows the number, size and shape of each chromosome as seen during metaphase of mitosis. With adults, the karyotype is done by using the nuclei of cultured white blood cells. With infants, a procedure known as amniocentesis is performed. During this procedure, the physician withdraws some amniotic fluid from within the placenta which contain cells sluffed from the fetus.

The white blood cells or fetal cells are incubated or cultured in chemicals which stimulate mitosis. A second chemical stops their growth at metaphase. The nuclear contents are photographed and the chromosome contents are paired up. Sexing the fetus and checking for "birth" defects are common goals for fetal karyotyping.