Fuzzy Math & The Forgotten Month?

How many months pregnant am I?  I'm 21 weeks.  I've got two pregnancy books both of which disagree on what month I'm in.  I say 6 months, but every time I do the math I end up with some extra weeks at the end of the pregnancy.  Here's what I keep coming up with:

1 - 4 weeks = 1 month

5 - 8 weeks = 2 months

9  - 12 weeks = 3 months

13 - 16 weeks = 4 months

17 - 20 weeks = 5 months

21 - 24 weeks = 6 months

25 - 28 weeks = 7 months

29 - 32 weeks = 8 months

33 - 36 weeks = 9 months

37 - 40 weeks = 10 months

Does this mean women are pregnant for 10 months but someone forgot to inform society?  I've been trying to take belly photos every month and now wonder what to do at 37 weeks (two 9 month photos?).  

After some careful Internet research I've found the error of my logic.  Apparently I'm using a lunar calendar to track my pregnancy.  Lunar calendars follow one full moon cycle which is 4 weeks.  While the Georgian calendar, followed by most of humanity, says a month is 4.333 weeks (probably where the 9 months came from).  

If I calculated my pregnancy using the Georgian calendar my due date would be July 22, 2009--exactly 40 weeks from October 15, 2008.  Which conveniently computes to 9 months (+1 week):

November 15 = 1 month
December 15 = 2 months
January 15 = 3 months
February 15 = 4 months
March 15 = 5 months
April 15 = 6 months
May 15 = 7 months
June 15 = 8 months
July 15 = 9 months (my official due date per doctors)

Apparently Grandma was right (she often is).  She told me I was 4 months pregnant and I told her I was 6 months pregnant.  We were obviously using different calendars.  So Grandma, you are not losing your mind, I am 4 months pregnant though I feel and look about 6 months pregnant and could possibly be 6 months pregnant if we all started following the lunar calendar.