I was wondering this same thing today, so Im 19 weeks, that means Im starting my 5th month!
technically it works out like this
wks 1 -4 = month 1
wks 5-8 = month 2
wks 9-12 = month 3
wks 13-16 = month 4
wks 17-20 = month 5
wks 21-24 = month 6
wks 25-28 = month 7
wks 29-32 = month 8
wks 33-36 = month 9
wks 37-40 = month 10
so at 36 weeks you would have COMPLETELY month 9 and started INTO month 10
at 37 weeks you would be a FULL 9 months and ONE week.
Does that make sense?
I would assume so. Although many women say happy 9 months, you are really pg for closer to if not more than 10 months.