CelticMoon, don't worry. These scans can be so inaccurate for guessing weight & date, even when done by the BEST people. My DD was born 7 weeks early. A scan done 12 hours before she was born estimated her weight at 2400g. She was 1800g (4 lb)!

The early scans, 12 weeks or earlier, are better for estimating the date. That is because babies grow at very much the same rate during the first trimester. If this is your first baby, don't worry too much about your guesstimate date. I'd go by the confirmed or the 12 week scan, which are close to each other anyway.