Now I get it!

Hope this helps you out a bit; I know it really helped me to figure it all out.

First Trimester - Month 1: weeks 1-4; Month 2: weeks 5-8; Month 3: weeks 9-13
Second Trimester - Month 4: weeks 14-17; Month 5: weeks 18-21; Month 6: weeks 22-26
Third Trimester - Month 7: weeks 27-30; Month 8: weeks 31-35; Month 9: weeks 36-40

There you have it!