During amniocentesis, a needle is inserted transabdominally into the amniotic cavity (identified by ultrasound; Fig. 7.4A) and approximately 20 to 30 ml of fluid are withdrawn. Because of the amount of fluid required, the procedure is not usually performed before 14 weeks gestation, when sufficient quantities are available without endangering the fetus. The risk of fetal loss as a result of the procedure is 1%, but it is less in centers skilled in the technique.
The fluid itself is analyzed for biochemical factors, such as AFP and acetylcholinesterase. In addition, fetal cells, sloughed into the amniotic fluid, can be recovered and used for metaphase karyotyping and other genetic analyses (see Chapter 1). Unfortunately, the harvested cells are not rapidly dividing, and therefore, cell cultures containing mitogens must be established to provide sufficient metaphase cells for analysis. This culture period requires 8 to 14 days, and consequently, making a diagnosis is delayed. Once chromosomes are obtained, major chromosomal alterations, such as translocations, breaks, trisomies, and monosomies, can be identified. With special stains (Giemsa) and high-resolution techniques, chromosome banding patterns can be determined. Furthermore, now that the human genome has been se-quenced, more sophisticated molecular analyses using polymerase chain reaction (PCR) and genotyping assays will increase the level of detection for genetic abnormalities.
CHORIONIC VILLUS SAMPLING (CVS)
Chorionic villus sampling (CVS) involves inserting a needle transabdominally or transvaginally into the placental mass and aspirating approximately 5 to 30 mg of villus tissue. Cells may be analyzed immediately, but accuracy of results is problematic because of the high frequency of chromosomal errors in the normal placenta. Therefore, cells from the mesenchymal core are isolated by trypsinization of the external trophoblast and cultured. Because of the large number of cells obtained, only 2 to 3 days in culture are necessary to permit genetic analysis. Thus, the time for genetic characterization of the fetus is reduced compared with amniocentesis. However, the risk of fetal loss from CVS is approximately twofold greater than with amniocentesis, and there have been indications that the procedure carries an increased risk for limb reduction defects.
Generally, these prenatal diagnostic tests are not used on a routine basis (although ultrasonography is approaching routine use), being reserved instead for high-risk pregnancies. Indications for using the tests include: 1) advanced maternal age (35 years and older); 2) history of neural tube defects in the family; 3) previous gestation with a chromosome abnormality, such as Down syndrome; 4) chromosome abnormalities in either parent; and 5) a mother who is a carrier for an X-linked disorder.
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