Prenatal diagnosis is now available for hundreds of conditions, ranging from profound mental retardation and early death (e.g., trisomies 13 and 18, Tay-Sachs), to disorders that affect daily living and shorten life span but do not cause serious mental incapacity (e.g., CF and hemophilia), and includes disorders that cause serious mental and physical deterioration but do not begin until middle age (e.g., Huntington disease). For many couples, the option of prenatal diagnosis offers the potential benefit of enabling them to have children they might not otherwise have been willing to bear because of the fear of severe birth defects or serious genetic disorders. In the vast majority of pregnancies in which it is used, the availability of information from prenatal diagnosis relieves parental anxiety (Platt and Carlson, 1992).

The most common indication for women seeking prenatal diagnosis is advanced maternal age historically at age 35 years and older. The incidence of chromosomal abnormalities increases gradually with maternal age. Prenatal diagnosis in advanced maternal age is a form of genetic testing generally provided in the United States by obstetricians as part of prenatal care during pregnancy. In a policy statement, the American College of Obstetrics and Gynecology has determined that offering prenatal diagnosis is part of the medical ''standard of care" for women age 35 and older (ACOG, 1985). Pregnant women are increasingly being offered maternal serum alpha-fetoprotein (MSAFP) screening, which is a blood test that can indicate the possible presence of a neural tube defect (NTD) (an opening in the fetal brain or spinal cord), Down syndrome, and an array of other fetal malformations. In addition, some fetal malformations of the heart, kidney, urinary tract, and stomach can be identified prenatally by using ultrasonography.

Prenatal diagnosis has focused largely on chromosomal abnormalities in pregnancies in women of advanced maternal age, couples with a family history of genetic disease, MSAFP screening for fetal neural tube defects and Down syndrome, diagnosis of hemoglobin disorders and hemophilia, and some biochemical abnormalities (WHO, 1983; NERGG, 1989; Medical Research Council, 1991; Modell, 1992). Some experts estimate that as many as 2.1 million pregnant women are now screened annually in the United States using MSAFP to determine increased risk of fetal genetic disorders (Haddow et al., 1992; J. Haddow, personal communication, 1993). Prenatal diagnosis is available for those couples identified through carrier screening as high risk. Rapid advances in genetic testing technology, especially with the advent of DNA testing (see Chapter 3), make it increasingly likely that a panel of prenatal genetic tests will soon be available, providing information on a large number of genetic disorders in the fetus—all at a relatively low cost (see Chapters 3, 4, and 8).

Chromosomal, biochemical, and increasingly, DNA-based laboratory methods are used for detecting genetic disease in the fetus (see Box 2-2). There are some important additional health risks associated with obtaining samples for anal-

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