Working out the dates

I am quite sure my GP was slightly over zealous with how far along I am and I think I am more in the fourth than fifth week based on various calculations I have done. Anyhow, we are seeing the Gynae on 27 May for the first scan and will then get a more definite due date. I suppose 5 days is neither here nor there really. I also just realised I am going to be heavily pregnant right through our height of summer here. Oh my, its going to be a long hot heavy summer for me!