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Computing the probability of both your twins having Down Syndrome (idealized model)
Because the blood tests cannot distinguish between the two foetuses, the results are difficult to interpret. Yet they tend to be communicated in the same manner as results for single foetus pregnancies - which I personally find confusing. What is intended here is a calculation to estimate the probability of giving birth to zero, one or two Down Syndrome children in a twin pregnancy after a positive blood test for Down Syndrome has been received. There are many caveats, but let's proceed...