Prenatal diagnosis had its beginning in 1966, when Steele and Breg showed that the chromosome constitution of a fetus could be determined by analysis of cultured cells from the amniotic fluid. Because the association between late maternal age and an increased risk of Down syndrome was already well known, their report led directly to the development of prenatal diagnosis as a medical service. Prenatal diagnosis has already been referred to in the context of many specific genetic disorders.

The purpose of prenatal diagnosis is not simply to detect abnormalities in fetal life and allow termination of pregnancy when the fetus is found to have a defect. Rather, it has the following objectives:

• To provide a range of informed choice to parents at risk of having a child with an abnormality.

• To provide reassurance and reduce anxiety, especially among high-risk groups.

• To allow couples at risk of a child with a specific defect, who might otherwise forgo having children, to begin a pregnancy with the knowledge that the presence or absence of the disorder in the fetus can be confirmed by testing.

Unfortunately, in a small proportion of cases the fetus is found to have a serious defect. Because effective prenatal therapy is not available for most disorders, the parents may then choose to terminate the pregnancy. Some pregnant women who would not consider termination nevertheless request prenatal diagnosis in order to reduce anxiety or prepare for the birth of an abnormal child. The question then is whether the request is justified if invasive techniques with an associated risk of fetal loss would be required. In practice the use of prenatal diagnosis by invasive techniques, even when termination of pregnancy is not an option, appears to be on the increase because the risks are low and because many believe that parents are entitled to the information.

Few issues today are as hotly debated as elective abortion, but despite legal restrictions in some areas, elective abortion is widely used. Among all elective abortions, those performed because of prenatal diagnosis of an abnormality in the fetus account for only a very small proportion, less than 2 percent. Without a means of legal termination of pregnancy, prenatal diagnosis would not have developed into the accepted procedure that it has become. Nonetheless, it is important to stress that, in greater than 98 percent of cases, the findings in prenatal diagnosis are normal, and parents are reassured that their baby will be unaffected by the condition in question. Among all prenatal diagnoses, the proportion culminating in abortion is thus very small.

Prenatal diagnosis requires the collaboration of a number of disciplines: obstetrics; ultrasonography; clinical genetics, including assessment, diagnosis, and genetic counseling; and laboratory sciences, including cytogenetics, biochemistry, and DNA analysis. Because of the complexity of integrating these functions, prenatal diagnosis is usually arranged by referral to a multidisciplinary program, in which genetics plays an essential part.

In North America, about 8 percent of all Pregnancies meet the criteria for prenatal diagnosis by amniocentesis or chorionic villus sampling. Both amniocentesis and chorionic villus sampling are invasive procedures associated with a small risk of fetal loss. Whereas amniocentesis or chorionic villus sampling is available to only a small percentage of pregnant women, a combination of maternal serum alpha-fetoprotein assay and ultrasonographic scanning can be used for fetal evaluation in low-risk as well as high-risk pregnancies because both are noninvasive and without risk to the fetus.

Maternal serum alpha-fetoprotein assay can help to identify fetuses at increased risk of neural tube defects and other disorders. Ultrasonography, in addition to its function in assessment of gestational age and fetal growth, enables geneticists and obstetricians to diagnose a number of morphological abnormalities, many of which are genetic in origin, at early gestational ages.