A = Has Down syndrome
B = Mother is between 40-44 years old
C = Mother is between 15-19 years old
Total Births = 4,265,996
15-19 year olds gave birth to 435,427 babies
40-44 year olds of age gave birth to 105,476 babies
Probability of having a Down’s child if 17 is 1 in 1200=0.000833333333
Probability of having a Down’s child if 44 is 1 in 30=0.0333333333
P(B|A)= P(A|B)P(B)/P(A) = 0.033 * (105,476/4,265,996) / 0.00125 = 0.65
P(C|A)= P(A|C)P(C)/P(A) = 0.00083 * (435,427/4,265,996) / 0.00125 = 0.068
It was never likely that Bristol gave birth to a child with Down syndrome, even if births among girls her age are more than four times as common as women of Sarah Palin’s age.
I guess the moral of the story is that while certainly not optimal, and not something I recommend, it is better to have a baby at 17 than at 44.
[Washington Post (pdf)]
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