Trimosomi 18 test

So I had it and came back with a 3% chance my baby boy has it and they gave me the option for blood witch is 99.9 % accurtate or an amino witch is 100% I choose the blood because I'm already high risk because of a hemorrhage I have between my uterine wall an the placenta , my Ob agreed on my choice, actually suggested it cause there's no misscarige risk, but my question is what are the chances of my baby really having trisomy 18