As far as I know, Trisomy 18 is basically a genetic disorder with onset before birth. The thing is baby who is "Trisomy 18" positive probably can't survive the pregnancy. Even if he/she could make it, there is a 90% chance that he/she will pass away before the 1st birthday.
Anyway, you should not be too worried as the blood test result is good. May God bless you and your little one.