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TABLE 1. Incidence of Down syndrome in live-borns and fetuses in relation to maternal age.

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

Source: Thompson et al. (3).