S.A.
in addition to all the other examples of false positives, i wanted to share my story. i got all false negatives and my son was born with Down syndrome. he is the love of our lives, family and friends. he is doing really well and in a typical preschool classroom. you may want to talk with a family or two that have children with these diagnosis to get a feel for how you would feel about having a child with special needs. personally, before and during my pregnancy i didn't know if i would be able to handle it if something went "wrong" but at the same time i was not going to abort. well something did go "different" and i wouldn't change a thing for the world. and i think you will find most other families with a child with Ds feel the same and feel so blessed to have that child. (and i say "blessed" as a figure of speech because i am not religious so that is not where these feelings are coming from. i hope that makes sense.)
good luck, do everything you can to relax about the situation. it really will be ok no matter the outcome. PLEASE contact me if you would like to talk. now or after you get more concrete results. hugs!