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GENETIC DISEASES
A large number of laboratory methods have become available for the screening of the embryo for detection of genetic anomalies. In many instances, such a diagnosis requires obtaining cells from the embryo. In many instances, this is achieved by virtue of an invasive techniqe called amniocentesis. During the second trimester, amniocentesis, is still the most widely applied technique in detection of aneuploidy in the fetus. An alternative strategy is the first-trimester chorionic villus sampling. This method is more ideal for pregnancies which are at a high risk for having an embryo with aneuploidy, inborn errors of metabolism and monogenic disorders. Biopsy of the placenta is considered in the second and third trimesters of pregnancy. Fetal blood sampling by ultrasound-guided cordocentesis has also been successfully used in the diagnosis of fetal infection and for rapid karyotyping. On the other hand, biopsy of the skin or liver are only rarly used. Fetal cells may also be obtained from the maternal circulation, however, determination of the utility of these cells in the diagnosisof genetic diseases has only recently begun. In contrast to these invasive techniques, other laboratory tests in common use, are non-invasive. In the second trimester, these tests utilize the maternal serum for quantiating the amount of such factors as AFP, hCG, and uE3. Prenatal serum screening for Down's syndrome is currenlty based on measuring the concentration of alpha-fetoprotein, unconjugated estriol, and human chorionic gonadotropin in the maternal serum between 15 and 22 weeks of pregnancy. These measurements, in conjunction with the age of the woman, provide tools to estimate the risk of having an ambryo with the Down's syndrome. By using these parameters, about 60% of Down's syndrome pregnancies can be identified. In addition, it has been estimated that the detection rate can be increased by 5% to 10% if ultrasound is used to determine the gestational age. In the May 9 issue, 1996 of New England Journal of Medicine, Aitken et al reported that in the second trimester, measuring inhibin A in the maternal serum, in combination with measurements of alpha-fetoprotein and the (beta) subunit of human chorionic gonadotropin, significantly improved the rate of detection of Down's syndrome. The serum concentration of inhibin A was 2.06 times the median value of the amount found in women with normal pregnancies Prior to gestation, inhibin is produced by the ovary. In addition to the ovary, during pregnancy, placenta also synthesizes this hormone. However, it is not yet clear why the amount of inhibin increases in the serum of women who have embryos with Down's syndrome. The new panel of tests should provide better assessment of the risk of pregnancies with the Down's syndrome. References:
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