They can make you terribly nervous and they don't even realize it. I know its crazy to say this - try to relax and just think good positive thoughts! every time you think something could be wrong replace the thought with something very happy about the baby....
MANY tests that you get from Dr.'s etc. not just ultrasounds but MRI's etc. for many other things have a huge chance of being false. My Dr. Told me about my weak looking u/s and then said look these things are not always accurate and I said I KNOW - because I have had my share of Mri'!! Some are correct and some are not - I once read that mri's are only about 60% accurate but Dr's look at the results as if they are absolute!
So hang in there - and try to think positive thoughts and smile really big to replace that creeping bad thought that can cross your mind - it helps me.
please keep us posted!!! Thinking of you -