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Conditions Screened

Prenatal screening provides a woman with her individual risk of having a child with
  1. Down syndrome,
  2. trisomy 18 and
  3. open neural tube defects.
It does not screen for all chromosome abnormalities, so some may be missed.
Each woman starts with an a priori risk of having a child with a chromosome abnormality based on her age at the time of delivery.  Screening adjusts this risk, either lowering or raising the risk based on the analytes measured in the blood and/or nuchal translucency measurment.
Risk of Chromosome Abnormalities in Liveborn Infants at Term by Maternal Age

Maternal Age (yrs)

Risk of Down Syndrome

Risk of ANY Chromosome Abnormalities

20

1/1,650

1/530

21

1/1,650

1/530

22

1/1,430

1/500

23

1/1,430

1/500

24

1/1,250

1/480

25

1/1,250

1/480

26

1/1,175

1/480

27

1/1,110

1/450

28

1/1,050

1/430

29

1/1,000

1/420

30

1/950

1/390

31

1/900

1/390

32

1/770

1/320

33

1/625

1/285

34

1/500

1/240

35

1/385

1/180

36

1/300

1/150

37

1/225

1/125

38

1/175

1/100

39

1/135

1/80

40

1/100

1/65

41

1/80

1/50

42

1/60

1/40

43

1/50

1/30

44

1/40

1/25

45

1/30

1/19

46

1/23

1/15

47

1/18

1/11

48

1/14

1/9

49

1/11

1/7

Adapted from:
Hook EB, Cross PK, Schreinemachers DM,  Chromosomal abnormality rates at amniocentesis and in live-born infants. JAMA 1983; 249: 2034-38.
Hook EB. Rates of chromosomal abnormalities at different maternal ages. Obstet Gynecol 1981; 53: 282-85.