Software-based Systems
A software program developed in Germany from 1995 to 2001, Perimon is a stricter variant of the rhythm method. It requires a greater period where unprotected sex is not allowed when used to avoid pregnancy, designating a maximum of 10.5 days each cycle as infertile. If 7 of those infertile days occur when a woman's is menstruating, this means there are only 3.5 days of non-menstrual infertile days per cycle.
No clinical studies have been done to determine effectiveness, but the program's developers claim a perfect-use failure rate of 4% per year. The Perimon software requires a paid subscription.
Several Web-based implementations of the cycle method are known. Examples can be found at Menstruation at the Open Directory Project.
Read more about this topic: Calendar-based Contraceptive Methods
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—Ruth Benedict (1887–1948)