Dates!! Measuring Please Help!

So I went in on May 4 baby measured 5 weeks 3 days went back 8 days later it measured at 6 week 1 day, shouldn't  a week and a day later, woulldnt the baby be 6 weeks and 4 days? Or am I wrong? The doc had me at 6 weeks 1 day, I'm getting so anxious about this pregnancy! Any ideas??