Quick question..

Doctor said yesterday that I was 5 and a half weeks. However I worked out from the day I conceived that I'm 4 weeks 5 days. So when I have my first scan will they be able to tell exactly how many weeks I am and give me a due date? Won't they think that the babies measuring less than it should be when it's actually how far gone I really am? This is my first so I don't really know anything as you can tell lol