This calculator might be of interest to you if:
  1. you are expecting twins and
  2. received a "positive" test result for Down Syndrome.
But I am not a doctor (not the right kind, anyway) so you should seek medical advice.

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...

User inputs

The calculation requires your age: (please adjust).

We will also need a probability of identical twins equal to (please adjust). This should be a reasonable estimate of the probability that your twins are identical, not fraternal. In other words, based on previous examinations (and not taking into account the blood test, which would be circular), roughly what probability did your obstetrician assign to your twins being identical? Please enter a number between 0 and 1. (The default setting of 0.1 is very roughly the population incidence of identical versus fraternal twins. However it is usually possible for a medical professional to form an opinion in your individual case.)

We will assume Down Syndrome odds of 1 in (please adjust). This is the number that was provided in the integrated test. (If your test was "positive" it generally implies the odds are 200/1 or less, but the terminology "positive" seems largely meaningless without the number itself).

Once you have adjusted these inputs, please .