I posed a comment earlier on the breakdown of weeks during pregnancy and here it is again.
Your First Trimester:
1st Month = Weeks 1,2,3,4
2nd Month = Weeks 5,6,7,8
3rd Month = Weeks 9,10,11,12,13
Your Second Trimester:
4th Month = Wks 14,15,16,17
5th Month = Wks 18,19,20,21
6th Month = Wks 22,23,24,25,26
Your Third Trimester:
7th Month = Wks 27,28,29,30
8th Month = Wks 31,32,33,34,35
9th Month = Wks 36,37,38,39,40