How accurate is dating ultrasound?

Today I had my first midwife appointment, and because they weren't able to find the heartbeat with a doppler they sent me for an early ultrasound. My dates have me at 11w0d, but the ultrasound puts me at 11w1d. Both of my boys were large (9.5 pounds) at birth. I'm just curious how accurate the ultrasounds are at this point, because I think it would make my sway stronger if the ultrasound is right, because our second attempt would have been too late (I swayed for a girl). Any ideas?

Not that accurate. It's the difference of just 1 millimeter.

Scans are around 7 days error margin at that stage, the further the pregnancy progresses, the less accurate for dates. I would work on your dates if you are sure of them

Thanks! It doesn't really matter because my babies are always late anyway... But I was just hoping my sway was a teensy bit stronger :). All the doubts are plaguing me until we find out in two more months...