Ms Tan: If i remember correctly, for the oscar i did for #1.
There will be several sets of results, 1 base on background (age, habits, etc), 1 base on measurements (Thickness of liquid @ back of bb's neck, whether bb has an obvious bridge at nose, etc) taken off baby, 1 base on the blood test. Each will have it's own risk ratio. There are some who don't do well for certain combi of the ratios, so need to do the amionic test, where they extract the water from the waterbag to test. But this test got high risk, so most gynaes won't recommend as first option.