21yrsold. normal u/s but high risk NT scan

Hi
This is my first child due on november 28 2011 ( 14weeks ). I took the NT scan/blood test. My scan was all good, there was a nasal bone which she said childen with DS dont have and the fluid in the neck was all well under and she was happy. But my blood test came back high with free B-hCg; 3.360MoM and PAPPA-A; 0.380MoM which leaves me at a risk off 1;78.
I am 21 yrs old with my partner only 23. Neither of us smoke aswel. We are todally freaking out due to why so low? for my age group anyways and no family history of DS. Why was my blood so high but the scan was all good?
Where considering having the anmio needle just to get mentally prepared. Just wanted to get everyones advice, Im so young and non smoker why so low? :(
Thank you