Yeah they can be off. I got one today for a variety of reasons and at 32 weeks (exactly) mine was weighing in at 3.28 lbs (less than half an ounce off for "average" babies at this gestation). Before we even began the doctor told me they are crazy off at this point and so long as they are within 2 weeks of your edd and a pound or so of the "average" then they are doing really well (they being the tech and the machine).
I know I make small babies (my biggest to date was 6 lb 13 oz) and if I did my math right this one will be in the same range as my other 2. I wouldn't fret it. Honestly, that seems like a nice sized baby to me! I'm always surprised when they weigh less at this point. I don't know why.
Just as "proof" how off they can be: I had an early ultrasound in order to set up patient status at my backup and so I could get a gender scan with no problems. This was within the first trimester so a realitvely more accurate scan. Based on head measurement only (why they only used that is beyond me
) I was due earlier than my chart says. There was no physical way that was possible. At my gender scan she was measuring pretty dead on with some measurements off a day or so. This scan she's due almost 2 weeks later (though weight matches up with my current gestation). That's with her head firmly lodged in my pelvis and her feet basically touching my diaphraghm (fundal height measuring consistent with 32 weeks as well).
So to sum up:
Early scan within first 13 weeks- earlier due date for being "bigger"
Gender scan at 22ish weeks- consistent with my due date
Scan today at 32 weeks exactly- weight is correct but other measurements put me 2 weeks less than I am.
And like a pp said: dr.'s are trained to find issues. There are just too many variables and with such cramped quarters you can't really expect legit numbers.
(FWIW DD2 was SHORT....barely 18 inches at birth..her height has often dropped her into lower percentiles while her weight is average. It balances out.