OK...heres my take on this...

There are 52weeks and 12 months in a year. 52/12 = 4.33 (ie there are 4.33 weeks in a month).

So....

4 weeks 2 days = 1 month
8 weeks 5 days = 2 months
13 weeks = 3 months
17 weeks 2 days = 4 months
21 weeks 4 days = 5 months
26 weeks = 6 months
28 weeks 1 day = 6 & 1/2 months
30 weeks 2 days = 7 months
32weeks 3 days = 7 & 1/2 months
34 weeks 5 days = 8 months
36 weeks 6 days = 8 & 1/2 months
39 weeks = 9 months