CAN DOCTORS BE WRONG?!

Hey girls, okay so I took a digital pregnancy test on the 18th of May this year and it told my I was 1-2 weeks pregnant. Than on the 23rd or 24th of May so roughly a week later I went and got my bloods and a test done at my local doctor. It came back somewhere between 4-5 weeks which is what i thought as when your egg is fertilised you are 2 weeks give or take, pregnant. So my conception date is around the 5th or 6th of May. Since than I have gone for a 20 week scan and I was 20+ 5 weeks when I got it done as that's the only time they could fit me in. And he was measuring on average around 21+ 3 weeks. My due date it the 26th of January 2018. So my question is, all these dates and bloods and tests, they can't all be wrong hey? Because I'm just paranoid ATM as I keep having dreams and it's doing my head in! 😩