Genetic Testing
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What is Down syndrome?

Down syndrome is one of many chromosome disorders.  Chromosomes are the genetic blueprints that determine how a baby will develop.  Chromosome disorders occur by chance when a baby receives too few or too many chromosomes at conception.  Down syndrome occurs when a baby receives an extra chromosome 21.  This disrupts development and causes birth defects and mental retardation.

What is genetic screening?

Screening tests are blood and or ultrasound tests that estimate the likelihood for the fetus to have certain types of birth defects, including Down syndrome and Trisomy 18.  A normal result does not necessarily mean that no abnormalities are present.  They do not guarantee a normal baby.  Screening tests always have a certain amount of false positives (an abnormal test in a baby with no birth defects) and false negatives (a normal test in a baby that has birth defects).

Who should have genetic screening?

Screening is offered to all pregnant women who are between 11 to 18 weeks pregnant.  Because they are at a higher risk of having a baby with chromosome abnormalities, the American College of Obstetricians and Gynecologists recommends that all women over age 34 or with a family history of chromosome abnormalities be offered chorionic villi sampling or an amniocentesis for chromosome testing.  These tests detect 99.9% of all chromosome abnormalities.  In contrast, screening detects 60 to 95% of Down syndrome, Trisomy 18, and Trisomy 13.  Five to ten percent of babies with these chromosomal problems will not be detected with screening alone.

How and when is genetic screening done?

There are two ways to do genetic testing.  If you choose to undergo genetic screening, you will only need to have one of these tests performed:

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First trimester ultrascreen: This test is done between 11 and 13 6/7 weeks and consists of 1) a special sonogram to measure the nuchal translucency (NT) – the amount of fluid behind the neck of the baby and 2) blood testing to measure protein leels (B-HCG and PAPP-A).  This test detects more than 90% of Down syndrome, Trisomy 18, and Trisomy 13.  This test does not detect spinal defects.  Therefore, further testing with a sonogram will be done at 18-20 weeks to detect spinal defects.

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Triple Screen: This test is done between 16 and 18 weeks of pregnancy.  It measures three different  chemicals in the blood.  No sonogram is necessary.  It will detect 60 to 70% of babies with Down’s syndrome and 80% of babies with spinal defects. 

Am I required to have genetic testing?

NO.  The decision whether to have the screening tests performed is your personal decision.  If these tests show that your baby is at an increased risk for a chromosome abnormality, it does not mean a problem has been diagnosed, only that further evaluation, usually with an amniocentesis, is indicated.  This can cause a lot of anxiety as well as some extra expense.  These are reasons why the decision to have genetic screening is left to the patient, rather that just doing it on all patients.

                                CVS                                                     Amino                                                  Term

Age

DS

ALL

 

DS

All

 

DS

All

20

 

 

 

1/1230

 

 

1/1923

 

25

 

 

 

     890

 

 

   1205

 

30

 

 

 

     690

 

 

     885

 

31

 

 

 

     650

 

 

     826

 

32

 

 

 

     560

 

 

     725

 

33

 

 

 

     450

1/200

 

     592

1/333

34

 

 

 

     350

   167

 

     465

   250

35

1/238

1/110

 

     270

   125

 

     365

   200

36

  175

     83

 

     210

   100

 

     287

   167

37

  133

     64

 

     166

     83

 

     225

   125

38

  100

     48

 

     129

     67

 

     177

   100

39

    75

     37

 

     100

     53

 

     139

     83

40

    56

     28

 

       73

     40

 

     109

     67

41

    42

     25

 

       61

     33

 

       85

     50

42

    36

     16

 

       47

     25

 

       67

     40

43

    24

     12

 

       37

     20

 

       53

     33

44

    18

       9

 

       29

     17

 

       41

     25

45

    13

       7

 

 

     13

 

       32

     20

46

    10

       6

 

 

     10

 

       25

     15

47

      7

       4

 

 

       8

 

       20

     12

48

      6

       3

 

 

       6

 

       16

     10

49

 

 

 

 

       5

 

       12

       7

 

 

Last modified: March 26, 2014