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The chance of having a baby with Down's syndrome is related to the age of the mother, approximately as follows:-
The levels of chance associated with having a Down's syndrome pregnancy in relation to a woman's age are shown in the table below:
Woman's age (years) | Chance as a ratio | %Chance |
Below 20 | 1:1600 | 0.067 |
20 | 1:1500 | 0.066 |
30 | 1:800 | 0.125 |
35 | 1:270 | 0.37 |
40 | 1:100 | 1.0 |
45 and over | 1.50 and greater | 2.0 |
If the mother of an affected child is under 36 then the chance of a further child being affected is said to be about 1 in 100.
Note however that the absolute numbers of mothers having children with Down's syndrome is greater in the younger age groups; this is because the total number of mothers in these groups is much higher.
For more information then see https://view-health-screening-recommendations.service.gov.uk/downs-syndrome/
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