i dont know about calculation as i never get tested for it reason being my mum was told with my sister she was ds even offerd her abortion well tahnk god my mum doesnt belive in abortions as my sister was perfectly healthy
and ds children are just as special as any other children :)
Idk how they figure it out but i had a blood draw and ultrasound and the end result was 1:10,000 that my baby would have downs. Sorry i couldn't be more helpful.