Why do I have to have an amniocentesis and not a nuchal scan to check for down's?

Diagnosis/Screening. The nuchal fold measurement is a marker for down syndrome and is used in screening algorithms to assess risk for downs. However, it is not a diagnostic test. The diagnostic test to definitively tell if you are carrying a baby with down syndrome is the amnio or cvs depending on gestational age. These test collect fetal cells that can then have the chromosomes counted.
Don't know. Why is a 30 yo having an amniocentesis suggested in the first place? Standard practice usually offers amnio to mothers who will be 35 yrs during the pregnancy. Is there a prior history of chromosomal abnormality which places you at higher risk? If so, then your doctor feels the definitive nature of the amnio warrants the 0.5% risk of post amnio miscarriage vs the merely suggestive nuchal scan.