Figuring out due date?

Spring Fever

Hi! I am cautiously going to try to make myself comfortable over here. I had my 1st u/s on Monday and the RE said I was 5 weeks 4 days, due on 8/8. I had a different due date in my head based in the ivf calculators I found online. The only one I found that matched the RE's due date is this one:  http://www.babymed.com/pregnancy-due-date-calculator?event=4

So how do they figure out ivf due dates? My 5 day transfer was on 11/18