Prenatal Testing (Genetics & Inherited Conditions)
Prenatal testing is administered to a large number of women, and the tests are becoming more informative. Some of the tests are only mildly invasive to the mother, but others involve obtaining fetal cells. Some are becoming routine for all pregnant women; others are offered only when an expectant mother meets a certain set of criteria. Some physicians will not offer the testing (especially the more invasive procedures) unless the parents have agreed that they will abort the fetus if the testing reveals a major developmental problem, such as Down syndrome or Tay-Sachs disease. Others will order testing without any such guarantees, believing that test results will give the parents time to prepare themselves for a special-needs baby. The test results are also used to determine whether additional medical teams should be present at the delivery to deal with a newborn who is not normal and healthy. Most often, prenatal testing is offered if the mother is age thirty-five or older, if a particular disorder is present in relatives on one or both sides of the family, or if the parents have already produced one child with a genetic disorder.
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Maternal Blood Tests and Ultrasound (Genetics & Inherited Conditions)
Screening maternal blood for the presence of alpha fetoprotein (AFP) is offered to pregnant women who are about eighteen weeks into a pregnancy. Although AFP is produced by the fetal liver, some will cross the placenta into the mother’s blood. Elevated levels of AFP can indicate an open neural tube defect (such as spina bifida), although it can also indicate twins. Unusual AFP findings are usually followed up by ultrasound examination of the fetus.
Other tests of maternal blood measure the amounts of two substances that are produced by the fetal part of the placenta: hCG and UE3. Lower-than-average levels of AFP and UE3, combined with a higher-than-average amount of hCG, increases the risk that the woman is carrying a Down syndrome (trisomy 21) fetus. For example, a nineteen-year-old woman has a baseline risk of conceiving a fetus with Down syndrome of 1 in 1,193. When blood-test results show low AFP and UE3 along with high hCG, the probability of Down syndrome rises to 1 in 145.
During an ultrasound examination, harmless sound waves are bounced off the fetus from an emitter placed on the surface of the mother’s abdomen or in her vagina. They are used to make a picture of the fetus on a television monitor. Measurements on the monitor can often be used to determine the overall size, the head size, and the sex of the fetus, and whether all the arms and legs are formed and of the proper length....
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Amniocentesis, Karyotyping, and FISH (Genetics & Inherited Conditions)
Amniocentesis is the process of collecting fetal cells from the amniotic fluid. Fetal cells collected by amniocentesis can be grown in culture; then the fluid around the cells is collected and analyzed for enzymes produced by the cells. If an enzyme is missing (as in the case of Tay-Sachs disease), the fetus may be diagnosed with the disorder before it is born. Because disorders such as Tay-Sachs disease are untreatable and fatal, a woman who has had one Tay-Sachs child may not wish to give birth to another. Early diagnosis of a second Tay-Sachs fetus would permit her to have a therapeutic abortion.
Chromosomes in the cells obtained by amniocentesis may be stained to produce a karyotype. In a normal karyotype, the chromosomes will be present in pairs. If the fetus has Down syndrome (trisomy 21), there will be three copies of chromosome 21. Other types of chromosome abnormalities that also appear in karyotypes are changes within a single chromosome. If a chromosome has lost a piece, it is said to contain a deletion. Large deletions will be obvious when a karyotype is analyzed because the chromosome will appear smaller than normal. Sometimes the deletion is so small that it is not visible on a karyotype.
If chromosome analysis is needed early in pregnancy before the volume of amniotic fluid is large enough to permit amniocentesis, the mother and doctor may opt for chorionic villus sampling (CVS)....
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Impact and Applications (Genetics & Inherited Conditions)
Until the development of prenatal techniques, pregnant women had to wait until delivery day to find out the sex of their child and whether or not the baby was normal. Now much more information is available to both the woman and her doctor weeks before the baby is due. Even though tests are not available for all possible birth defects, normal blood tests, karyotypes, or FISH can be very comforting. On the other hand, abnormal test results give the parents definite information about birth defects, as opposed to the possibilities inherent in a statement of risk. The parents must decide whether to continue the pregnancy. If they do, they must then cope with the fact that they are not going to have a normal child. When properly administered, the test results are explained by a genetic counselor who is also equipped to help the parents deal with the strong emotions that bad news can produce. Genetic testing also has far-reaching implications. If insurance companies pay for the prenatal testing, they receive copies of the results. Information about genetic abnormalities could cause the insurance companies to deny claims arising from treatment of the newborn or to deny insurance to the individual later in life.
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Further Reading (Genetics & Inherited Conditions)
Bianchi, Diana W., Timothy M. Crombleholme, and Mary E. D’Alton. Fetology: Diagnosis and Management of the Fetal Patient. New York: McGraw-Hill, 2000. A resource for practitioners and a guide for parents. Illustrations, bibliography, index.
De Crespigny, Lachlan, and Frank Chervenak. Prenatal Tests: The Facts. New York: Oxford University Press, 2006. Designed to provide prospective parents with objective information about prenatal testing. Discusses the reasons for prenatal diagnosis and describes normal and abnormal fetal development. Provides a general overview of the available prenatal tests, as well as more specific information about testing during various stages of pregnancy.
Evans, Mark I., et al., eds. Prenatal Diagnosis. New York: McGraw-Hill Medical, 2006. Begins with a review of genetic and reproductive risks and then discusses various prenatal diagnostic procedures, including ultrasound, amniocentesis, and fetoscopy. Covers laboratory diagnostics, management of prenatal problems, types of fetal therapy, and the ethical, legal, and social issues surrounding prenatal diagnosis.
Heyman, Bob, and Mette Henriksen. Risk, Age, and Pregnancy: A Case Study of Prenatal Genetic Screening and Testing. New York: Palgrave, 2001. Provides a detailed case study of a prenatal genetic screening and testing system in a British hospital, giving perspectives of pregnant women,...
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Web Sites of Interest (Genetics & Inherited Conditions)
Association of Women’s Health, Obstetric, and Neonatal Nurses. http://www.awhonn.org. Offers pages for education and practice resources, as well as legal policy.
Family Doctor.org, Prenatal Diagnosis: Amniocentesis and CVS. http://familydoctor.org/online/famdocen/home/women/pregnancy/fetal/144.html. Family Doctor.org, a Web site sponsored by the American Academy of Family Physicians, includes a page describing amniocentesis and chorionic villus sampling, with links to additional information.
Internet Pathology Laboratory, Prenatal Diagnosis. http://library.med.utah.edu/WebPath/TUTORIAL/PRENATAL/PRENATAL.html. The site, cosponsored by Mercer University School of Medicine and the University of Utah Eccles Health Sciences Library, is an online source of information about pathologic findings associated with human diseases. The site’s page on prenatal diagnosis describes the benefits of prenatal testing, the various types of tests, and techniques for pathologic examination. It also provides an overview of fetal-placental abnormalities, with links to additional information about medical conditions.
March of Dimes. http://www.marchofdimes.com. This site is searchable by keyword and includes information on the basics of amniocentesis and chorionic villus sampling and articles on how the two procedures relate to genetics....
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