I was wondering the same thing. My second trimester blood draw covers some things they've already tested (Down Syndrome, etc), but it also tests for neural tube disorders like spina bifida and anencephaly, which the midwife said weren't covered in the first trimester tests. I don't know what the Maternit21 covers, but it sounds like maybe they can't do the neural tube tests til second trimester. She said they'll combine the first and second trimester results for a quad screen (https://americanpregnancy.org/prenatal-testing/quad-screen/), which is just a little more thorough.