MaterniT21 vs. first tri screening

MAdams728

Sorry if this has been covered, but I havent quite found a solid answer for my question.  

I am planning on having the first tri screening done, but from what I'm reading the MT21 test seems to be a better option?   If your doctor will offer the test, why isnt it better to skip the nt scan/bloodwork and go straight to the MT21 which seems to be more accurate?  I am planning on bringing this up to my doc at my 9 week appt.  Before I do bring it up, I wanted to make sure I wasnt missing something obvious that explains why the first tri screening is the first choice.  Is it just because the MT21 is new and not widely available, or always covered by insurance?   

TIA!