Maternal age (years)

Incidence

At birth

At amniocentesis (16 weeks)

At chorionic villus sampling (9-11 weeks)

15-19

1/1250

-

-

20-24

1/1400

-

-

25-29

1/1100

-

-

30

1/900

-

-

31

1/900

-

-

32

1/750

-

-

33

1/625

1/420

-

34

1/500

1/325

-

35

1/350

1/250

1/240

36

1/275

1/200

1/175

37

1/225

1/150

1/130

38

1/175

1/120

1/100

39

1/140

1/100

1/75

40

1/100

1/75

1/60

41

1/85

1/60

1/40

42

1/65

1/45

1/30

43

1/50

1/35

1/25

44

1/40

1/30

1/20

45 and over

1/25

1/20

1/10