I am 39 years old, i am 17 weeks pregnant now. at 14 weeks i took maternit21 and it was positive for trisomy21. what are my chances of faulse positive?

Very unlikely: Very unlikely - but you do need further testing if you have more questions - please ask.

Cell-free fetal DNA : (cffDNA) circulates freely in the maternal bloodstream from 7 weeks gestation till ~ 2 hours after delivery. Multiple methods are used to test it for a variety of genetic disorders, including aneuploidies, disorders with abnormal #'s of chromosomes. Though both sensitivity & specificity of MaterniT21 testing are quite high, you should have been offered definitive invasive genetic testing.