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This method uses relatively complicated calculations to determine the fertile days.
A woman who wants to use this method must first document her periods in a menstrual diary for one year.
The length of the shortest and longest cycles is determined and entered in a formula developed by doctors after analysis of a large body of data.
The number 17 is subtracted from the shortest menstrual cycle, e.g.
27 days (shortest cycle) minus 17 days = 10
The number 13 is subtracted from the longest menstrual cycle, e.g.
32 days (longest cycle) minus 13 days = 19
In this example the days from the 10th to the 19th day of the cycle would be regarded as the fertile days. On these days sexual intercourse is not allowed or other methods of contraception must be used (e.g. diaphragm, cervical cap, chemical methods, condom).
This method is very unreliable. There are many factors that can influence the rhythm of the cycle and make the fertile phase unpredictable (natural early or late ovulation, stress, travel, change of climate, unaccustomed physical exertion).
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