Technically a month is longer than 28 days (4 weeks) -so every three months there is one extra week. (= 52 weeks in a year)

There are 13 weeks or so per trimester - plus 1 extra somewhere ?

Some books say 3rd trimester begins at end 27 weeks - others start of 27th week

As such we are 'pregnant 9 calendar months and 1 week' (but as you said really less 14 days from first day of last AF cycle)

I go by weeks or 6 months is 26 weeks - next 3 months makes you 9 months or 39 weeks - last week is week 40.

24 weeks is 5 calendar months and 2 weeks so not actually 6 months.

Yes it is very confusing :-k