is the scan accurate?

Hi All,
Things start to calm down and I really enjoy being pregnant.I just want to ask a question: I had a scan at 14 weeks and it said I am due the 19 feb.my doctor said most of the time is accurate. is it true? i heard also that it could be two weeks, do they mean two weeks before 19 feb or one week before or one week later?
I didnt plan the pregnancy and I dont recall my last period date
lots of love
Dinia
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