Chromosome analysis . obtained by amniocentesis or Chorionic Villous Sampling is diagnostic for Trisomy 21, other trisomies & monosomies. It is offered if the calculated statistical risk of DS from screening by blood tests & fetal ultrasounds + maternal age at EDD & family history reaches a cut-off point, often 1:270. This calculated risk method has an 80% detection rate & a 5% false-positive rate. .
Depends on the test. Here the OB 's often offer an AFP blood test to moms in low risk groups or as an initial screening for ds. It is not conclusive either way but may be considered adequate reason to do the riskier cvs test if the parents want the info. The genetic test from the cvs is going to identify the chromasome pattern for the baby. That is reliable. Newer screening tests may be available in some areas.