Rhythm Method

This method is based upon observation of Ogino, in 1930 in Japan and Knaus, in 1933 in Austria.
This method is based on an observation that the ovulation in females takes place 2 weeks before menses. Now as we know that the menses cycles in women may not be on precise day every month, the calculation of the safe period is done on the basis of duration of 12 previous cycles. First note down the shortest and the longest cycles during the last 12 cycles.
The first fertile day will be minus 18 days from shortest cycle.
The last fertile day would be minus 11 days from the longest cycle.
You can discover examples how to calculate your safe days.