How to calculate period of gestation (POG) manually

1. know the last menstrual period (LMP)
2. calculate from date of LMP until the date of today

eg: LMP : 14.4.2011

       14.5
       14.6
       14.7
                   this 3 month is 13 week = 13/52

so today is 20.7.2011, minus 14 from 20 (20-14) = 6day

so, plus the week and the day = 13/52 + 6/7..

52 is total of week per year
7 is total of day per week.