Frequency of Down Syndrome
Per Maternal Age

 

Age (years) Frequency of Fetuses with Down
Syndrome to Normal Fetuses
at 16 weeks of pregnancy
Frequency of Live Births of
Babies with Down Syndrome
to Normal Births
15 - 19 ---- 1 / 1250
20 - 24 ---- 1 / 1400
25 - 29 ---- 1 / 1100
30 - 31 ---- 1 / 900
32 ---- 1 / 750
33 1 / 420 1 / 625
34 1 / 325 1 / 500
35 1 / 250 1 / 350
36 1 / 200 1 / 275
37 1 / 150 1 / 225
38 1 / 120 1 / 175
39 1 / 100 1 / 140
40 1 / 75 1 / 100
41 1 / 60 1 / 85
42 1 / 45 1 / 65
43 1 / 35 1 / 50
44 1 / 30 1 / 40
45 and older 1 / 20 1 / 25

The numbers are approximated and rounded.
Using this data, geneticists have set the number separating low-risk from high-risk at 1 / 250.

Why is there a difference in frequencies between 16 weeks and time of birth?
Because of the spontaneous miscarriages of pregnancies with Down syndrome between these times.

For information on risks of more detailed situations (such as translocation or mothers
who have had previous babies with Down syndrome, see Dr. Paul Benke's essay
on Risk and Recurrence of Down syndrome.

Reference for the above table: Hook EB. JAMA 249:2034-2038, 1983.


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