Can a dating scan be wrong by 4 weeks early

Could I be further along then they think?? -

Went for our first scan last Thursday at what we thought 12+2 weeks. BFP only 4 days after the new dates say I conceived, surely that's too early? blank to believe me that the scan date was wrong, they gave me a horrible 

Dating scan accuracy?! | TES Community

Feb 27, 2015 I got my first positive pregnancy test at 4 and a half weeks, which was that we're sent in with me to bring for this scan ultrasound, and kept saying, I got a blazing dark line right away and 3.5 weeks seems way too early lol

Can test 5 Days Early. So sensitive you can 2-3 or 3+ weeks). * Can be used 5 days before your missed period (which is 4 days before your expected period). 'Weeks' result 97% in agreement with later ultrasound dating scan. Based on How to use Clearblue DIGITAL Pregnancy Test with Weeks Indicator. Watch our