Frequency of Down Syndrome Per Maternal Age

Down Syndrome Health Issues ^

[Sopater]

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.

Return to Prenatal Testing for Down Syndrome

[Sopater]

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.

But with Prenatal Testing for Down Syndrome, how many are actually aborted rather than miscarried? If they were all due to miscarriages, the numbers would also seem to indicate a higher likelihood for miscarriage in the younger women than the older women.

[GOP Poet]

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[HighlyOpinionated]

Statistics like this are skewed.

Add another column and list the total number of babies born in each age group. Then add another column that lists the percentage of Down Babies IF the number of births in each age group were the same. (Kind of like statistics on oil prices in "current dollars.") You'll find that the % is very close to the same all the way up the scale, it's just that women 40 years old do not have as many babies as women half their age.