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Innermost Secrets / Innermost Living / Body Clock
Skip Navigation LinksTesting for Down Syndrome
Page modified at: 10/07/2010

Traditional Screening for Down's Syndrome

Various tests have been developed to try to identify those women at greatest risk of having a chromosomally abnormal baby such as Down's Syndrome, who could then decide whether or not to proceed to amniocentesis. The TripleTest or Barts Test at about 15 weeks into the pregnancy detects about 70% of Down's Syndrome pregnancies based on about 1 in every 25 (4%) women having the blood test ending up with a high risk result (i.e more than 1 in 150 at term) and then proceeding to amniocentesis. This means it misses 1 Downs Baby for every 2 it picks up! It cannot identify pregnancies at increased risk of Trisomy 18 (Edward Syndrome) or Trisomy 13 (Patau Syndrome).

Disadvantages of Serum Screening (Triple Test)

The test is less efficient in younger women with a detection rate of only about 30% and in women over 35 there is a high likelihood that testing will place them in a high risk category warranting amniocentesis. This means unecessary amniocentesis procedures and miscarriages which could have been avoided. This has been highlighted in a recent article in the Guardain.

The Guardinan

NHS failure on Down's screening - click the link to read the article!

The blood test is not done until 15 weeks and therefore decisions about amniocentesis are made at 16 weeks with final results and decisions about the pregnancy made at 18 weeks. At this late stage the woman can feel the baby's movements and the pregnancy is almost half way through. If termination is considered for an abnormal result it is usually by a medically induced birth, which may mean up to 2 to 3 days in hospital.

The test is not valid for women with multiple pregnancy and cannot be relied upon in women who have had a previous baby with a chromosomal abnormality such as the common type of Down’s Syndrome (Trisomy 21).

If all is well then the woman will have had to wait until nearly 18 weeks into the pregnancy to have the reassurance that the baby does not have Down’s Syndrome if her blood test shows a high risk result.

What is the maternal age related risk of having a baby with Down Syndrome

The risk of Down Syndrome increases with advancing maternal age but about half of babies with Down Syndrome are in mothers under 35 years old and therefore ALL women should consider having a screening test.

Maternal Age Risk of Down Syndrome

Maternal Age at term

Risk of Down Syndrome

20

1:1450

21

1:1450

22

1:1450

23

1:1400

24

1:1400

25

1:1350

26

1:1300

27

1:1200

28

1:1150

29

1:1050

30

1:940

31

1:820

32

1:700

33

1:570

34

1:460

35

1:350

36

1:270

37

1:200

38

1:150

39

1:110

40

1:85

41

1:70

42

1:55

43

1:45

44

1:40

45

1:35

46

1:30

47

1:30

48

1:30

49

1:30

50

25

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