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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 
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.