Well, I'm 4m pregnant and in my 5th month, same with weeks: I'm 18w pregnant and in my 19th week. That's because you don't get a week 0: the first day of your period is 1d pregnant, ovulation is 15d pregnant: easy, 0+1 and 2+1, right? So weeks 0 and 2. No. Weeks 1 and 3; there is no "week 0".

Does that make sense? HOWEVER, if the book reads like 20w "woo hoo, half-way now" then I'm willing to bet it didn't start right at the beginning and probably missed conception (like most books - it's the websites that start at w1!) so in my books I'm looking at w18.