Testing for Down Syndrome

Cytogenetic analysis looks at the number and structure of a person's chromosomes. This test, which can be performed on a blood sample, is the test used to definitively determine if an individual has Down syndrome.

Risk of Down syndrome and other chromosomal abnormalities increases with maternal age. Adapted from Evans, 1989.

amniocentesis removal of fluid from the amniotic sac surrounding a fetus, for diagnosis

Maternal Age

Risk of Down Syndrome

Total Risk for all Chromosomal Abnormalities

20

1/1667

1/526

21

1/1667

1/526

22

1/1429

1/500

23

1/1429

1/500

24

1/1250

1/476

25

1/1250

1/476

26

1/1176

1/476

27

1/1111

1/455

28

1/1053

1/435

29

1/1000

1/417

30

1/952

1/385

31

1/909

1/385

32

1/769

1/322

33

1/602

1/286

34

1/485

1/238

35

1/378

1/192

36

1/289

1/156

37

1/224

1/127

38

1/173

1/102

39

1/136

1/83

40

1/106

1/66

41

1/82

1/53

42

1/63

1/42

43

1/49

1/33

44

1/38

1/26

45

1/38

1/21

46

1/23

1/16

47

1/18

1/13

48

1/14

1/10

49

1/11

1/8

Prenatal diagnosis for Down syndrome (testing for the condition during pregnancy) is possible. Chromosome studies can be performed on fetal cells collected via chorionic villus sampling (CVS) at ten to twelve weeks of pregnancy or by amniocentesis at fifteen to twenty weeks of pregnancy. Because of the link between the mother's age and the chance of Down syndrome, prenatal diagnosis for Down syndrome and other chromosome conditions is routinely offered to women thirty-five and older. Whether to pursue prenatal diagnosis is a personal decision that can only be made by the parents. see also Birth Defects; Chromosomal Aberrations; Meiosis; Mosaicism; Nondisjunction; Prenatal Diagnosis.

Angela Trepanier and Gerald L. Feldman

Bibliography

Evans, Mark I., et al. Fetal Diagnosis and Therapy: Science, Ethics, and the Law. Philadelphia, PA: JB Lippincott Co., 1989.

Gardner, R., J. McKinlay, and Grant R. Sutherland. Chromosome Abnormalities and Genetic Counseling, 2nd ed. New York: Oxford University Press, 1996.

Nussbaum, Robert L., Roderick R. McInnes, and Huntington F. Willard, eds. Thompson & Thompson Genetics in Medicine, 6th ed. Philadelphia, PA: W. B. Saunders, 2001.

Pueschel, Siegfried M., ed. A Parent's Guide to Down Syndrome: Toward a Brighter Future, 2nd ed. Baltimore, MD: Paul H. Brooks Publishing, 2001.

Internet Resource

Cohen, William I., ed. "Health Care Guidelines for Individuals with Down Syndrome: 1999 Revision." Down Syndrome Quarterly 4, no. 3 (1999): 1-15. <http:// www.denison.edu/dsq/health99.shtml>.

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