To amend my first statement... blood tests on MOM can be done to rules out most generic disorders (i.e down syndrome, ect) ...and there are NO risks to baby. I would talk to your OB about that and if the blood test results show an increased risk of problems then do the amnio.
Have to? Is your Dr. requiring it? I've always refused because there is an increased risk of miscarriage after the procedure. And unless you plan on terminating the pregnancy if something is wrong w/ the baby, I've always seen them as sort of a pointless risk. Ask why you're being recommended for one.