My background risk (based purely on age) for T21 was 1:576 and for T18/13 1:993

My adjusted risk (based on age plus the blood tests and nuchal fold measurements) for T21 1:11504 and for T18/13 1:19845

These are the notes on my ultrasound letter. I don't know what the highest numbers you can get are but the cut-off between high and low risk groups is 1:300.