7 week u/s and dating question

Native+Girl

We had our 7 week u/s this morning and everything went great! We got to see AND hear the hb, which we weren't expecting. It was the most amazing sound in the world and brought DH and me to tears. The hb was in the 140s and our LO was measuring 6w6d so that's pretty good. 

I do have a question. If I had my IUI on a Saturday, would that make Saturdays or Fridays the start of a new week. Like is today 7w0d or 7w1d? When I enter in my IUI date into due date calculators, it says I'm 7w0d but my doc has always said "::week #:: and 1" on my Saturday appts. Not a big deal, but it's the difference between the due date being my grandma's bday or the day after so I was just curious.