5weeks 2days

Our fertility clinic rang today to book our 7 week scan, which was a small shock as we thought we were only about 3 and a half weeks along but acording to their maths we are about 5 weeks and 2 days along. Our 7 week scan is on the 4th October. I have to be honest i dont understand their maths, my egg harvest was on the 29th august and we had 2 5days embryos put back on the 3rd of sept? By any chance does anyone here understand these dates. Not stressing or worrying about it, i just think it would be nice to understand it. Many thanks in advance for any input.