the screening is being conducted; (2) social controversy surrounding the fact that no treatment exists for most of the conditions for which the screening is being done and that prevention of the disorders involves termination of pregnancy; (3) the critical importance of a qualified MSAFP testing laboratory (and essential criteria for quality control in such laboratories); and (4) the need for adequate education of physicians or other health professionals, as well as for facilities and personnel for follow-up of abnormally high or low MSAFP results, as well as for those whose initial results are false positive, and patient education (ASHG, 1987, 1989). In addition—with the use of folate supplements prior to or early in pregnancy—the risk of NTDs may be reduced (Laurence, 1990; Scott et al., 1991); research is continuing (McPartlin et al., 1993).

Several new research techniques have been developed that may hold promise for prenatal diagnosis in the future, including genetic tests on fetal cells isolated from maternal blood and genetic tests associated with preimplantation diagnosis. These techniques are still experimental, however, and have not been thoroughly studied and evaluated for safety, reliability, and effectiveness. Despite their early stage of development, some of these techniques are already beginning to be used in clinical diagnosis (see Box 2-3).

An experimental test that is already widely used, called the "triple-marker screening test," has been reported to increase the effectiveness of MSAFP screening alone in predicting increased risk of a variety of fetal trisomies in pregnancy (Crandall, 1992; Haddow et al., 1992). The new screening test involves the analysis of the combined measurements of AFP, plus serum human chorionic gonadotropin and unesterified estriol (a component of estrogen), in maternal blood. These tests can be used between approximately 15 and 20 weeks of gestation. This triple-marker screening is reported to be more reliable than MSAFP alone in identification of pregnancies at increased risk of Down syndrome (60 percent detection versus 20 percent detection, respectively), and of some cases of trisomy 18 (a severe form of physical disability and mental retardation). Triple-marker screening is also reported to improve identification of twins, correct determination of gestational age, and improve identification of women at increased risk of late-pregnancy complications (those with unexplained high levels of MSAFP) compared with MSAFP alone. As with MSAFP alone, confirmatory amniocentesis and ultrasound are required.

Critical Issues in Prenatal Diagnosis

Many people believe that prenatal diagnosis can assure them of having a baby healthy in every respect (Wertz, 1992). However, most disorders cannot now be diagnosed prenatally, and there are few possibilities for primary prevention of most congenital abnormalities. DNA technology offers the prospect of gene therapy and genetic engineering at some time in the future, but the more immediate application of this technology is the expansion of the number of disor-

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