intrauterine therapy or from routine use of ultrasonography (Purkiss et al., 1988; Nicolaides et al., 1992). The low sensitivity, specificity, and predictive value of ultrasound examinations for congenital abnormalities, as well as the time and expertise needed for the procedure (Gowland, 1988; Manchester et al., 1988), have also dampened scientific enthusiasm for routine ultrasound screening for congenital abnormalities in the United States (U.S. Preventive Services Task Force, 1989; Holtzman, 1990). Although prenatal ultrasonography is not considered "standard of care" for routine obstetrical practice in the United States by the American College of Obstetrics and Gynecology, it has generally become a routine part of obstetrical practice. However, standards for ultrasound equipment and for the training and certification of personnel have not yet been developed. Some ultrasonography experts (Filly et al., 1987; Gowland, 1988; Manchester et al., 1988; Townsend et al., 1988; Goldstein et al., 1989; Lancet, 1992) report a substantial error rate (as high as 10 percent wrong diagnoses with both false positives and false negatives) among obstetricians and some centers in reading ultrasound images, and large variations in image quality associated with equipment and its maintenance.

Ultrasound is a tool commonly used by primary care providers in routine obstetrical practice where an individual practitioner will see few abnormalities. Consultation with genetics professionals and highly specialized ultrasonographers is essential when fetal abnormalities are suspected because of the inherent difficulty of interpretation and the need for experience in recognizing abnormalities. When ultrasound is performed by highly skilled operators, the sensitivity of this screening device in detecting congenital malformations can be as high as 90 percent (Manchester et al., 1988).

ysis for prenatal diagnosis. Many of the concerns that exist regarding the specificity or sensitivity of particular tests apply to prenatal diagnosis, as well as newborn screening and heterozygote detection (see above). The key difference is that the mother must undergo an invasive procedure that puts the fetus at risk in order to obtain this information. The risks to the fetus of a prenatal screening or diagnostic procedure need to be weighed in decisions about prenatal diagnosis, along with the risk of the birth of an affected child. There must be a much lower tolerance for false positive test results in prenatal diagnosis because of the undue anxiety that can be raised by the uncertainty (Juengst, 1988), as well as the potential for erroneously aborting an unaffected fetus.

A variety of techniques are currently in use for obtaining fetal cells for genetic analyses, including midtrimester amniocentesis, chorionic villus sampling, and the less common percutaneous umbilical blood sampling (PUBS). Ultrasonography is used for guidance of the needle in all three techniques. It is also used for fetal visualization and gestational dating, and can detect gross fetal anomalies (see Box 2-2).

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