I asked my doctor, just to be sure (because I want to know and would LOVE to get an extra week out of the way) and he looked at me like I was crazy.
Oops, sorry to make your doctor look at you crazy!
It's not getting an extra week out of the way, as such (unfortunately). It's just saying you're beginning your 14th week. I guess it's like saying once you've completed 13 weeks full weeks (when you reach 13+0) you are then starting your 14th week - on 13+1 you are one-seventh of the way through your 14th week etc.
A similar example would be that we are in the 21st century - we havent had 2100 years yet, only 2010, but still, as soon as we get to 2000 (20 centuries) we *start* the 21st one, even if we're only one year into it we are still in the 21st century.
Sorry, I really don't mean to labour the point, but I just wanted to have one more go at explaining!
I'll stop now...