diagnosis, involves removing cells at the blastocyst stage and using PCR to amplify the segments of DNA (one from each parent) containing the part of the disease-related gene that contains the disease-causing mutation of interest. If the disease-causing mutation is absent, the embryo(s) is then implanted in the woman's uterus (Simpson and Carson, 1992). Prefertilization determination of whether an ovum contains a specific disease-causing mutation has also been accomplished by polar body analysis (Verlinsky and Pergament, 1984; Verlinsky and Kuliev, 1992). If the ovum does not contain such a mutation, in vitro fertilization is carried out, and the resulting embryo is implanted in the woman's uterus; this technique also involves the use of PCR. These techniques are still considered experimental and are discussed further in Chapter 2.

Recombinant DNA technology has also added new techniques for detecting changes in the number of chromosomes present in a fetus's cells (aneuploidy). The technique involves fluorescent in situ hybridization (FISH) of chromosomespecific probes with cells obtained prenatally (Klinger, 1992). It is far less time-consuming and labor-intensive than the earlier technique of karyotyping, which nevertheless is capable of detecting a much wider range of disorders than FISH.

Empiric observations have revealed that when a woman is carrying a fetus with an extra chromosome (i.e., trisomy), she is likely to have altered concentrations of alpha-fetoprotein (AFP), chorionic gonadotropin, and estradiol in her blood (so-called triple-marker testing for increased risk of abnormalities in the fetus) (Haddow et al., 1992). This has permitted the development of maternal blood tests for these substances as initial screens of pregnant women for increased risk of such fetal abnormalities. Confirmatory tests are necessary for actual diagnosis.

New techniques for prenatal diagnosis are also being evaluated involving fetal cells that are usually present in and can be isolated from the maternal circulation in the first trimester of pregnancy. These techniques are still highly experimental (Bianchi et al., 1991; Elias et al., 1992). Although this technique is noninvasive of the fetus, the scientific, ethical, legal, and social issues associated with other forms of prenatal diagnosis are still present (see Chapters 2 and 4).

Many of the techniques discussed thus far involve gene changes (mutations) that are—with rare exceptions (i.e., mosaicism)—usually present in all cells of the body, having occurred as germline mutations and inherited from one or both parents. On the other hand, there is now evidence that as cells become cancerous they undergo a number of nongermline, noninherited, somatic cell mutations in different genes (Fearon and Vogelstein, 1990). Preliminary work suggests that the presence of these somatic cell mutations may be detected (via PCR) by analysis of fluids in which mutated cells might be shed (e.g., fecal matter in the case of colon cancer), which would serve as an early warning of the development of malignancy (Sidransky et al., 1992).

Advances in ultrasound have resulted in its use to detect structural changes in the fetus, some of which may be associated with genetic or chromosomal abnormalities. Although not a genetic test in itself, ultrasonography is used as an ad-

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