For most expectant parents, awaiting the birth of a baby brings at least some anxiety. Will everything turn out okay : Will labor and delivery proceed normally? Will the baby be healthy? But for those who know they have a higher-than-average risk of conceiving a child with genetic problems, a nine-month wait can be agonizing. Fortunately, there are some steps couples can take to alleviate their fears. One of these is prenatal testing.

Conducting a genetic test on an unborn baby requires that some of the baby's cells be retrieved. Doctors usually use one of three methods to sample fetal cells, each of which caries some risk for the developing fetus. Amniocentesis is an effective method of collecting cells found in the fluid surrounding the developing baby, but has a 0.5 percent risk of causing miscarriage. Chorionic villus sampling, also with a 0.5 percent risk of causing miscarriage, removes fetal cells found outside the amniotic sac. Fetal cell sorting is noninvasive, unlike the other two methods, but requires that fetal cells be painstakingly identified in and collected from the mother's blood.

Once the fetal cells are collected, there are a number of tests doctors can perform to screen for genetic diseases. One of the oldest and most common genetic tests doctors use today looks not at individual genes but at whole chromosomes. Dozens of diseases are caused by problems with either the number or structure of one or more of the 23 pairs of chromosomes. Down's syndrome, for example, is caused by the presence of an extra chromosome number 21. Other diseases, such as Alagille syndrome, result from chromosome translocations, in which pieces of chromosomes are lost or swapped during meiosis.

Chromosome problems like these and others can be identified in the genetic material of fetal cells using a technique called karyotyping. A karyotype is an organized arrangement of all the chromosomes found in one of the patient's cells. It is prepared by chemically interrupting the process of cell division during metaphase, when the chromosomes are most visible. The chromosomes are then stained and photographed, and the photographs are enlarged. Each chromosome image is cut out and arranged according to its size and shape. From this arrangement, doctors can easily identify abnormalities like extra or missing pieces of chromosomes.