Mathematically correct calculation of trimesters...

01 to 04.5 weeks - Month 1
04.5 to 09 weeks - Month 2
09 to 13.5 weeks - Month 3
13.5 to 18 weeks - Month 4
18 to 22.5 weeks - Month 5
22.5 to 27 weeks - Month 6
27 to 31.5 weeks - Month 7
31.5 to 36 weeks - Month 8
36 to 40 weeks - Month 9

1/3 of 40 weeks = 13 weeks & 2 1/3 days long

20 wks is 50% or 1/2
21 wks is 52.5%
22 wks is 55% or 55/100
23 wks is 57.5%
24 wks is 60% or 6/10
25 wks is 62.5%
26 wks is 65% or 65/100
27 wks is 67.5%
28 wks is 70% or 7/10
29 wks is 72.5%
30 wks is 75% or 3/4
31 wks is 77.5%
32 wks is 80% or 8/10
33 wks is 82.5%
34 wks is 85% or 85/100 17/20
35 wks is 87.5%
36 wks is 90% or 9/10
37 wks is 92.5%
38 wks is 95% or 95/100
39 wks is 97.5%
40 wks is 100% or 3/3 or 10/10 or 100/100